About two weeks before my test date I stopped memorizing bio, orgo and all that and just started doing full length dats. You can study bio and everything else till the cows come home, but you'll always be missing things and you won't realize the things you're missing until you get stumped on individual questions while doing full length tests. If I got a question wrong on a full-length test, I never forgot that fact later. In my experience, you can study a particular area a lot, and still not know how to answer a question on it unless you know what types of questions can be asked. By far, kaplan is the most useful for the 4 or 5 full-length tests they have on their computers. I was amazed at people who took the kaplan class and never used the computers- what a waste! I think some people go wrong in studying for the dat by not doing questions/full-length tests. You can't just sit down at a table and study for the dat by reading stuff.
Also, I would not suggest using supplemental materials for studying for the DAT in sections like bio and orgo. You are guaranteed to get questions on the dat that weren't covered in your kaplan course - i got some random question on bones in the leg which sucks for me because I'm a biochem major and never took anatomy or any of that. But all the info you can study is just way too much - you'll never be able to do it so don't waste your time. You can't get all the questions!!