If you have played an instrument long enough, you will no doubt get to a point where you feel you are proficient. Maybe not as good as you once hoped you would be, but at least good enough for whatever gig you find yourself in. I made the mistake of resting in this point for a couple years time and didn't grow at all because of it. Those are definitely years I wish I had back.
Well how can you stretch yourself in your main instrument? My first recommendation would be to find a lesson plan that you have not tried before that will challenge you. Get a method book of some sort that is a notch above your level and dig in. Don't be afraid of trying something a little tough. You want to be the best player you can be? Well, running over that same pentatonic scale for the 10,000th time isn't gonna get you there. Sometimes after beating your head against the wall for a while and not getting any results, you should try a different approach.
My main instrument is the guitar. So here is one recommendation. If you know how to read some standard notation and want a book that will challenge you to learn so much, check out A Modern Method for Guitar - Volumes 1, 2, 3 Complete
If you are a drummer, perhaps something like 4-Way Coordination: A Method Book for the Development of Complete Independence on the Drum Set
But really whatever instrument you play, I'm sure there is something out there what will do the trick. I would suggest that you read reviews of any text before you purchase it, because there is a lot of garbage out there. Spend a little time searching for the right book and dive in.
What method books or skill books do you recommend for your particular instrument? Leave a comment and let us all know!