1. Don't be too reluctant to spend your cash. If there's an item (jewellery typically but rarely also armor) with an essential skill attribute or one with an active skill you're investing into and/or want a temporary boost, then it can be a good idea to buy it, especially if it's a unique item as they are typically better than the random generated items.

2. Early on it's good to buy food to add to your stocks rather than potions as they're cheaper and can consume food/drinks whilst on your way to your next group of goblins or skeletons early on in the game.
Even while you're fighting, if you're not losing HP too fast you can gradually stablise or steadily increase your HP by consuming food whilst fighting (probably most effective with a mage or ranger build).

3. Vendors restock after each time you level up (excluding unique weapons/armor/jewellery and malachite gems).
Most vendors's prices drop about 25% when you mindread them, so do this before purchashing costly items (if you'd prefer to spend a little EXP for considerable less gold, usually worth it early game as you don't have much gold to start with.

4. Most skill caps can be lifted later on in the game up to 15 (from 5-10-13-15, I think this is important to mention as I haven't played many RPG's but none of them had a raisable skill cap so I thought 5 points is it and you're maxed).

5. Keep in mind some quest items can be equipped so don't sell that 2nd favourite amulet or ring just yet if it turns out your favourite is a quest item (assuming you want to finish all quests).

6. Mindread, Wisdom and Lockpick are all valuable if not essential skills to invest into. Beware though, 5 Skill Points is the maximum cap for these Lockpick and the caps can't be lifted until you reach the Battle Tower.
Wisdom helps you to level up faster and gain extra EXP (but this depends on if you turn in your quests right away or leave them until the last moment) keep in mind you can always regain every skill point you've invested later on in the game and completely redistribute them however you like (go to #17).

7. Read through EVERY or at least most books as some contain helpful information as well as bits of the storyline. This goes the same for every quest you accept - click on each quest to see additional information.
I can't stress the last sentence enough. (I completed a quest that gives you the option to do 2-3 extra quests in a certain area - this was "Into a Bandit's Den" and had no idea you could go there for extra quests/loot before reporting the location to Lieutenant Louis and finishing that particular quest.)

8. Costly mindreads are usually worth it. They can allow you to do extra/hidden quests, spawn chests or items, give passwords to storages plus you extra stat/skill points. Just remember to quicksave before you talk to/mindread someone (if you decide you don't really need that mindread).

9. The moral, truthful and righteous path on a quest is not always (but can be) the most profitable/beneficial nor the one that always finishes a quest so choose your discretion. It just depends on whether you don't mind having a failed quest in your log having done the rightful thing.

10. Only fight enemies at the same level as you (if not higher assuming you've killed all enemies at a lower level than you) to maximise XP gains.

11. Play around with the game until lvl 5-7 however you like but start to think through and devise a plan of what build you've chosen and skills to invest into early on in the game.

12. Smash every crate, barrel, urn etc., search well and you may find hidden switches, levers and pressure plates that can lead to very valuable or useful items.

13. Generally, take EXP over gold in quests or some weapon/armour etc. item.

14. Keep a decent amount of gold early on (15-20k at least so you can buy particular (unique) weapons) but try to never take gold from quests, you can get plenty of gold easily later on around level 20. You can farm gold quick and easily later on in the game.

15. There is a limited amount of save slots you can have (but it's quite high). Nevertheless try to keep 1-2 saves maximum per level and use save slots as checkpoints. So if you had forgotten to do something before you fought a group of enemies (such as swap out mindread boosting items for wisdom boosting ones), finished a quest or whatever it is, so you can always revert back to that point. Stick to using the quicksave function mostly, but use the manual save slots before starting, finishing quests or tough fights.

16. Summoning skills are quite handy, summoning undead, demon and your creature you can create your own little army. (but summoning ghost which only heals you and only melee attacks if it needs to), Summon Mastery can add damage to all 3-4 creatures as well.

17. Distributed Skill Points aren't set in stone, you can regain all of them at once multiple times (for a cheap but doubling cost) later on in the game so don't stress too much about what to pick.

18. There is a very limited amount of Malachite Gems (refer to exploitable game mechanisms, section on how chest/ores function) and they are very useful later on in the game as you'll find out yourself but Malachite Ore is unlimited, so be sure to remember this and not get them confused.

19. Always carry a bow on you early on, especially in the earlier levels when you have low stat points, no skill points etc. you can take down a horde easily with a bow (just don't let yourself get hit too much, melee or ranged.

Although you can get an unlimited amount of ores and herbs, these first 2 mechanisms explained are still somewhat helpful.

1. Except for unique/quest items, chests generate random loot and a varied number usually 2-3 but up to 5-6 different items per chest so quicksave and load until you get something you like and remember to quicksave BEFORE you go up to/scroll over a chest with your cursor. (Most chests seem to not have predetermined items once your cursor has hovered over a chest for a certain time, but to be safe, don't hover your cursor over the chest before quicksaving a metre or two away.

2. This is similar for ores and herbs except you'll always get that specific type of ore from whichever type of ore it is as well as herb, the only difference is the amount you get as well as having a chance to gems for a decreased amount of ore pieces (which cost more and are typically worth it).
[Edit 1]: With this said, you should quicksave before going up to a malachite ore to mine so you can quickload if you don't get a malachite gem from each ore.

3. Talk to everyone, quicksave before you talk to practically everyone because different dialogue options can lead to different ways of finishing quests (which you can't go back on without a save beforehand), you can try different replies for different and/or extra dialogue (if you really like seeing every possible dialogue option like I do).

4. Put Quests off as long as you can, as there's plenty of enemies around and your EXP gains scales from what level you are vs. theirs, as it takes more EXP to level up the higher you get, you want to delay finishing quests as much as possible.

5. As Mindreading goes to an 'experience debt' if you're trying to maximise XP gains then use this to your advantage and try to do helpful mindreads very early on as soon as level 2 in Broken Valley, for example if you have no new skill points invested into mindread, mindreading Richard for 400XP will increase the amount you need to level up significantly. You could go for a Ranger build until maybe level 10-15 once you've maximised the amount of XP you can gain as it's probably the easiest combat build early on (just shoot, dodge and run away from melee enemies whilst shooting them).
The reason why I say it's a good idea to do this is because there's too many creatures in Broken Valley of certain levels that you won't be able to ideally take them all out whilst being the same level they are, and hence you'd be losing XP. Racking up a huge debt greatly slows down your level rate as you have to clear the debt before earning XP that will count towards your next level.

Bracelets, necklaces and earrings from a chest or quest have a very good chance of having a +1 to +2 skill attribute. Belt's and rings have a chance but I've only seen ones with +1 skill attribute and I think these were unique items anyway. Therefore it's a good idea to keep in mind you can get your skills from 3 to 6 skill points under your cap if you're going to be using them frequently. With that said, quicksave before certain quests (always do it anyway for every quest in general as certain quests although very few in number will always reward an item (usually those 3 types of jewellery) with +2 skill attribute points) that almost always reward 1-2 of these items until you find a bonus skill attribute you like on an item. Although I have to warn you, it can take from 1-30+ minutes to find a really specific skill attribute but that time is very well spent.

I recommend trying to get items with the Mindread, Wisdom, Lockpick and Master Herbalist skills (although the only item I've seen with +1 lockpick is Jagon's necklace). Wisdom is the most easiest/common skill to find, then Mindread by half or less than half of the times you find items with Wisdom attributes, Master Herbalist a little less common than Mindread and Lockpick being basically non-existent as far as I've seen.

The reason why I recommend these non offensive/defensive passive skills is because say you want max Mindread but would usually rather invest into other skills, if you get 3 or more different types of items all with +6 Mindread points total, you can basically get Mindread to 9 and only equip those items just before a mindread. You can have 3 or more items with +6 Wisdom total equipped whenever you're fighting or closing quests to get bonus EXP and if you manage to find 1-2, possibly even 3 (although I need to try getting 2 let alone 3 to confirm this myself) items with Master Herbalist, you can just equip all 3 and have Max Master Herbalist only when you're going to brew potions. And basically every skill point you have after you respec the first or second time can be invested into offensive or defensives skills, ones you'll only be using in fights.

Sidenote: weapons, shields and armour can have bonus skill points as an attribute but they're usually offensive/defensive skills, usually 1 but it's not uncommon to find ones with +2 points.