I do lots of different stuff... Lessons, books, utube, Scott , did a rockschool grade, play a set list , some band practice, counting to songs in the car, book cd and headphones on holiday, polish the bass, finger movements on the bass, read some theory, do scales arpeggios and 1/2and 4 finger position variations, now doing stuff on a 5 string over 2 octaves, GAS , Bass Bash, always buying bass books with CDs, one day I might do blues riffs, then rock, then country or some African stuff from some books that have unusual bass examples....
...if I can't be arsed to handle the bass I'll just read and listen to riffs etc... If I can't be arsed to do that I'll play something on the bass....if I can't be bothered to play ill do finger exercises
I'm no expert but for me its just doing some kind of bass related thing when I can :-)... It's all practice and knowledge
I've improved lately apparently :-)