I find it helps to group my categories by how mandatory they are to pay as well. It wasn't my idea, but I can't find the post I read it in right now either. Basically I have:
Bills (Things I'm committed to paying or I don't have a house/power/etc)
Savings (Rule 3 categories)
Discretionary (Clothing, a few other could splurge if I want to categories, and actually I put Groceries in here because you have so much control over it)
Recreation (Fun money!)
I start at the top and allocate generally towards the bottom. Obviously I put some money in Groceries before I worry about saving (eating is good!) but generally flow from top to bottom. So let's say I get halfway through the month and realise I've got an overspend in Groceries. I see a category with $600 available, but it just so happens to be Bills: Car Registration and Insurance. Ok, certainly not going to raid that... Let's start from the bottom. Oh, I've got $1000 in Car Replacement I could poach if I need to... You get the picture.
If your categories are set up in order of importance, you realise pretty quickly which categories you can raid. It keeps you from taking money out of something you need to cover a lumpy bill, and also helps you budget in order of importance.

