I'm a huge freaking sucker for adoptables, especially as a writer who can't draw their own characters.
Things that tend to draw people are unique concepts and obviously polished art. Closed species are hugely popular on devianart and separate you from the crowd of other people who are doing adoptables.
Regular old human adoptables are hard to come by, so that would be something different (I've been searching for three weeks with a general idea of what I want and still nothing >.<).
Offering custom adopts is also a good idea!
Definitely make sure you have a very clear TOS that includes your feelings on using your designs for commercial work as well as resale of adopts- so that somebody can't buy them from you and sell them for double the value, that happens.
A constant on deviantart as well is the concept of 'adopt auctions'- pick a starting price you think is fair for the amount of work you put in and then take bids. It's surprising how high they will get. Just make sure your terms of the auction are clear and that you keep to them- nothing is as annoying to me as making my bid and then having the artist ignore their own rules so I miss out on it or have to pay more.