Yeah, pretty much a standard problem - some CARs are fine and honest but many are unsurprisingly swayed by either their teammates/manager or just are ignorant of the LOTG. They don't always pay attention to the game so aren't in good positions and when they do flag it's often half-hearted (flag slung over shoulder seems a fave) or questionable. On the other hand, I know most teams have low expectations and as long as it isn't blatant cheating I'm prepared to go with what I have.
I talk to the CARS with the captains at the toss, and reiterate the Law re: it not being an offence to be an offside position, so I want them to be playing/attempting to play the ball before a flag. I also say I want a late right flag rather than a wrong quick one...more so the captains know my expectations. I do overrule - a clear signal and voice 'thanks lino!' works pretty well - and have had people refuse to carry on. If it gets really stupid then I'll go and have a word, along the lines of 'you know why I overruled that, don't you?' and, if they refuse to flag for me I will say to the manager that it's up to them, but chances are I'm not going to be in a position to judge offside and I won't give it if there's any doubt.
Personally I long for the days when there are enough refs and funds for 3 assts at every game, but I'm not naive enough to hold my breath. What would be good is for training to be available for all clubs to have a qualified CAR - but that would only solve part of the problem I guess.