Every day there is an opportunity to improve your track driving. If you are fortunate to be at a track the the opportunity to improve is obvious, but what about the other days?
Read a book, you will find an excellent one in the shop
Review old data or in-car video from previous races/tests, even if its old, critique it and learn from it.
Look up other drivers in-car videos on the net (here for example), critique them and learn from them.
Spend a few minutes practicing getting your mind focused. The more you do this the easier it becomes when you are in the assembly area prior to qualifying.
Do some exercise, the fitter you are the longer you can keep your lap times and concentration up when it matters.
In short, do something everyday, even if its just 5 mins and it will all add up.