The most valuable habit I've developed IMO is reinventing the wheel. When you face a new challenge with new concepts you're unfamiliar with, the best way to get a good grasp of the environment/tools/techniques is to rebuild it (them) from scratch. Example: I was spending a lot of time interacting with external REST API's (foursquare, twitter and other services) but never really understood the inner workings, so I just decided to build my own RESTful API for fun. It doesn't really help anybody but now I know what the request / response cycle is and how it works, how to pass headers, what they're used for, the actual meaning of response codes etc.

Oh and also, NEVER make any assumptions about how people will use your software