Similar to Marijke I try to use as much positive reinforcement as possible. When he does what I ask I thank him for doing what I asked. I find it very hard to get truly cross with him though so tend to tell him a lot how wonderful he is and how much I love him LoL.
Also, sometimes when he starts acting out because everything's gotten too much I don't yell at him for acting out, I praise him for managing so well and tell him how proud I am of him for doing so well the whole day and that I know he's tired but we will be home soon and then he can go and rest. For e.g. if we've been out for a number of hours, here there and everywhere, and he's generally been really well behaved, but then the car trip home starts to take just a bit too long and he starts with his tired moaning and crying etc, that's when I do that, because I can tell it's purely because everything's just gotten a bit much and a bit more than he can cope with.
Also, if he's done something he's not supposed to and I say "no, don't do that, get down" and he does, then I praise him for listening to me and getting down. Or "no, don't smack the cat, stroke him gently like this" and then he does, then I say "yes, that's nice, softly gently, that's nice" so he understands when he's doing things the right way.