I'm repeating some of what others have said, but here's my 2 cents worth:

1: It's a timing thing. I find them easier to do rolling to the right, and going counter-clockwise. Just pick the directions that are easiest for you. Starting from right side up, I go left, down, right, up and so forth.

2: Learn how to do a regular rolling circle smoothly first. Make it so that you can do the same circle, at the same altitude, same speed, and the same roll rate every time. Now, pull the power back some, and increase the roll rate. Continue doing this untill the plane has to pitch up to maintain altitude. Now you can practice getting slower, and lower untill you have it down.

3. Never do a harrier roll based on timing alone. Always be alert of the way the airplane is pointed, and which input will be required. You need to have this down to a reflex, however make sure that you dont get your reflexes out of sinc. When I was learning to do it, I cant tell you how many times I got rolling too fast and either missed an input, and then gave it 90 degrees or more too late, or just got out of sequence. If you get one step ahead or behind the airplane, then you can be in a mess of touble if you are low.

4. Don't be afraid to back off the ailerons, or give them more to make it comfortable.

5. Although not neccesary, a good knife edge mix helps alot.