I do the Shunamite diet, which is originally a British rat diet, but it's easy to make in the States, too. Here's a link to the basic principles of the Shunamite diet:
For my own version, here's a list of what I use for each category:
40% base food:
Harlan Teklad blocks
10% dog kibble:
Wysong Synorgon dried dog food (You want a kibble that has the meat listed first in its ingredients--this is actually somewhat difficult to find in most normal dog food brands.)
10% uncooked pasta:
tricolor spiral pasta
40% breakfast cereals:
Currently, I use a mix of puffed wheat, puffed rice, plain shredded wheat, cornflakes and Quaker rolled oats
The great thing about Shunamite is its adaptability--you can alter the ingredients depending on what kind of results you want. (For instance, for adult rats you might want to decrease the amount of dog kibble slightly; for overweight rats, slightly decrease the pasta; etc.) You can also change the types of cereal you'd like to use every so often to give them some variety.
I started out with my boys on a mix of Reggie Rat (which they had been used to before I got them) and Oxbow Regal Rat (which they hated). Ever since I've switched them to Shunamite, they LOVE their food and can't wait to get a fresh bowl of it in the morning! Their fur is also in much better shape, and overall they just seem more energetic and happy!
Though it is a bit pricey at first, in the end you'll save money. I mix up a batch of this big enough to fill a large rubbermaid food container, which lasts more than a month for my two boys, and I still have much more than half of almost all of the ingredients left over!!