#1. Due diligence, due diligence, due diligence, due diligence.
Before you buy, you need to verify everything. You can't take anything the seller says at face value. You should be able to re-recreate their books from scratch and authenticate any data. Also, you need to always have in the back of your mind: "What's the real reason they are selling this business? They'll tell you a reason, but oftentimes there's another motivating factor.
#2. What value can you add to the business? Could the business see a big jump in revenue, if you applied your marketing prowess? Is there some problem that the business has that you know how to fix?
For example, a friend of mine bought a local pie company, thinking that the pies could be very successful if sold nationally by mail. The previous owner couldn't figure out how to do this successfully. My friend figured out a process, and grew the business and it ended up being a great purchase for him.