- Stay together and cover each other
Humans can only win if they play as a team. Don't run off alone, always make sure no one is left behind, pinned down by a Reaver, or (being) taken away by a Sentinel. Depending on the situation, if someone died and respawned away from you, you should try to head in his/her direction (all the while staying safe).

- Don't stay TOO close together
Staying too close to each other is a good way to all get hit by a single attack. Leave enough space between each other so that when a vampire attacks he can only get to one or two of you, so that the others can all attack the vampire at the same time, killing him quickly.

- It's very important to watch each other's health
If someone's health is below 50%, the whole team should go to the nearest supply station so that everyone who needs to can heal up and refill their ammo supplies. If your health is below 50% you likely won't survive the next attack, and letting someone run off to a supply station on their own is obviously very risky. It's more important to stay alive than to get kills, since vampires can only regain full health by executing a defeated player.
While using the supply station you can (and should) look around and reload.
If the supply station close to where you're camping is empty, you better relocate to an other place close to a full supply station.

- Communicate
If you're low on ammo, say so in the team chat by pressing Y.
Even if you can't hit an enemy (or see it anymore), just shoot in its general direction to alert your team.

- If you know you're about to die, quickly use your AoE abilities.
You might be able to damage or even kill a vampire after your death, or prevent them from snacking on your body, and provide protection for your teammates.

- Don't let a vampire regain health
Don't let a vampire with low health get away, its health will partially regenerate, its skills will cool down, and it'll become a much bigger threat. Don't get seperated from your team though. Don't keep chasing the vampire for too long. And of course, don't let vampires drag away or execute any humans.

- Don't be an easy target
Keep moving around as much as you can, between trees, pillars, etc, breaking the line of sight between you and any reavers that might be preying on you.

- Don't stand too close to corners and walls
You want to be able to roll and/or run away when attacked.

- Don't forget humans have melee attacks too
This is especially true for Alchemists (as they do damage to themselves when shooting a target that's too close) and for hitting Reavers after they've triggered Evasion.

- After respawning, don't always run towards your surviving teammates
After dying from a vampire attack, depending on how many teammates are still alive, how much health they have, and how many vampires you guys managed to kill, it's not always the best idea to run towards them. Chances are by the time you arrive they'll be dead and you'll be up all alone against multiple vampires, which is just giving them a free kill. Sometimes it's better to wait until your teammates are killed and respawned next to you. It's up to you to know when though.