The first step to future engine work is to not mess up my current game. That means doing a branch of my engine code to retain the current look for Tiki Lounge Blackjack. Anyone who isn't already using some sort of source control to help out this process is living in a dream world. Source control is good. Destabilizing your finished game is bad.
The second step is separating out all of the implementation specific source from the common code that could be shared across a DX and GL implementation for example. I've done a good job at abstracting things out that the amount of implementation specific objects is fairly minimal.
After that it's onto some general code cleanup around comments, removing any stale code that is in there, making sure my APIs are consistent around objects and doing a refactoring pass. I'm confident that I can shrink my code base by at least 10-15% with a refactoring pass due to duplicate code I've seen.
The final stage is document generation using Doxygen and updating my samples to use the new libraries. The docs and samples aren't really necessary as I'm the only developer using these bits but it forces me to clean things up properly. It also means that if someday down the road I find someone else to share dev tasks it won't be as ugly as bringing in someone in with no docs and ugly code.