Oh God, we have this too. Sometimes my palm itches to box DS's ears because I get so frustrated that I cannot leave them alone for one minute without him hitting DD, leaning on her, headbutting her, rolling on her. It wears you down, it really does.
I tried star charts (boy, that didn't work - he just went loopy once he'd acquired a star), time out for 2 mins, shouting, screaming, crying. And none of it worked.
I then re-read 'How to Talk'. Since then, we still have bad hours (usually prompted by tiredness/hunger), but we haven't had a full day of hitting/nagging. I think what the book does is help you to get into their heads better - if DS is being treated 'well' then he has less reason to be angry with DD. And if he's getting nice attention then he's less inclined to provoke the unpleasant kind.
I have also had Toddler Taming recommended to me by a friend who had this exact same problem with her DCs. She's lending it to me and I'll do an update once I've read it. I have to say that I'm not overly impressed with the title - DS can be a little wild at times, but I don't actually want to tame him - I just want him to stop being mean to his sister!
He did have a day when he pushed twice at a smaller child that we're friendly with. I have knocked that on the head by telling him that if I ever see him pushing/hitting a smaller child again, I shall just take him straight home where there will be no telly and no engines. I always follow through and he knows that so we've had no repeats yet. If we do, then I will stick to the threat.
FWIW, I think it's absolutely inevitable that DD will hit him back when she's big enough, and from what I've seen of younger siblings generally, I suspect she'll also hit other children. I am really not looking forward to that and hope that my policy of leaving immediately once there's been an incident will solve the problem.
Sorry about the essay - this has been a big problem and worry for us and I know it's still an ongoing task (we had a couple of incidents last night) but things are a lot better than they have been.