I wish I had enough to give to charity, MickS! That's an excellent idea. I've always been told 10%, though I will do that, or $75 per pay check, depending on which is highest that pay check. It means $150 goes into savings each month, which is the best system I've come up with so far.
I also find that having multiple bank accounts is helpful. I have my checking and two savings accounts. The Checking is the money I'm allowed to spend, I put all my bill money into one savings account and then the other I put my savings into. It has been very successful so far. I never go over and am never short on bills, unless I don't estimate enough for the power bill, but that's a whole rant on its own :)