Well the simple answer is just keep stretching, however I understand that once you’re quite flexible it becomes harder to find stretches that actually work…
One technique I found that really helps is resistance stretching. It’s based on the fact that muscles become flexible more easily when they are simultaneously tensed and stretched. This Youtube channel has a number of different stretches and explains everything behind it: http://www.youtube.com/user/flexiblestrength?blend=6&ob=0 Try giving that a shot, it’s helped me heaps!
In addition if you can already do the splits, but want to work on your oversplits, try placing your front foot on something a little higher. Start with a book or a cushion, and then gradually increase the height. Eventually you can get to the point where you can do splits between two chairs, or even better. The same can be done for the side splits, though you should elevate both legs at the same time and the same amount.
Finally, always make sure you are warm when you are stretching for the best results and to prevent injury! And also, make sure you hold your stretches for at least 30 seconds, because that’s the necessary length of time to increase a muscle’s flexibility.
Hope I helped! Best of luck :)