Very easy:
Singleplayer -> Choose 'Quick Race' -> 20 minutes with max. KI-level & aggressiveness, only BMW M2 as opponent, max. cars and start from last place -> Track: Suzuka (or any other track where it is hard to overtake or with lots of turns) -> Car: Any GT3 you like.

The race: Overtake them just on the straights and do it very close, in the turns stay very close behind them and just overtake when it's safe. When you are first, stop the car, let some cars pass and do the same again.

In 20 minutes I gain +2 SA-Rating. I got 40 or more trust in a race.

Today I did two races in open lobby (=Multiplayer) at Bathrust, also each race 20 minutes but with qualyfing. I earnd just +1 or 2 SA-Rating (in the lobby it showed 71 but in race 70. Back again in the lobby it was at 72). Very disappointing, because it was a very clean race, but not much overtakes like in the singleplayer method and it took 60 minutes (but I wouldn't belittle the experience you gain).

I also recognized in a 40 minutes Monza race with stormy weather, that I gained SA-Rating although at some point I was alone on the server. Maybe this will add some SA-points in total.

Just my 2 cents:
You don't win the race in Turn 1! A race has at least 7 or 8 laps!
If someone is faster than you, let him pass. Even if he is lapped (name is marked blue). It is a unnecessary risk.
Before Turn 1 break carefully and not at the last moment. Look what the guys in front and behind of you are doing. Try to drive foresighted (like in real life). If some is divebombing, don't turn in. Let him drive straight in the shadow realm and laugh. Just try to survive lap 1 with at least damage as possible and after that it will be good. Sure, if someone wants to crast you, there is not much you can do. But you can be clever and set a trap (if you try to overtake him and he wants to bump you from the side, break and see him loosing control).
My last 10 races were awesome, just with these simple tips. And I do gain positions. I start from P12 and finish with P4 (as example), because the guys in front of me are crahsing and I just drive safe (and fast).
One example: Some days after writing this guide I did a 1h Bathurst race. I finished qualifying with P12. As the race startet I let everyone pass and startet at green lights at P20 - another guy did the same. So green lights, no chaos at T1 but chaos at the straight. I drove carefully, avoided the crahsing cars, etc. and was P15. Then the hills... more crashes, more positions gained, etc. I just drove carefully but also fast, had 30 sec in damage, no pit stop because I also calculated the fuel accurate and finished P2! The guy, who did the same, finished P4. The distance to P1 was only 16 sec at the end.

Also a tip for choosing open lobby servers:
If you look below the server name, you will see something like this (Explanation behind):
FP1 14/15 (in white); Q15; R30 (this means 14 minutes practice left)
FP1 15; Q 1/15 (in white); R30 (this means 1 minute qualifying left)
FP1 15; Q 15; R 29/30 (in white) (this means 29 minutes race left)
FP1 15; Q 15; R 30 (in white) (this means race is over)

Also another tip for Qualifying:
If you leef the box/pit lane and start your first round, you are on your outlap. This round doesn't count (like you now). You will notice this guys as blue dots on the map. Guys who are marked grey are on their flying round/hot lap. Move when they (grey dots) are close to you and don't ruin their lap time, because you have nothing to loose but they.
Also look on the map, when you are in the box and choose the correct moment to start your outlap. Don't get in a "traffic jam" or don't hinder someone.