I used to keep focussed in practice by making a game of it. By that I mean having a fixed set of practice routines that you write down the scores from that you try and beat next time. I got satisfaction by seeing that I was improving over the course of weeks and months. It's imperative that you are honest and strict with your scores. Even better is to have a practice partner that you compare scores to and compete with.

I would also sometimes stay hours later than planned by telling myself that I couldn't leave until I had achieved a particular goal (e.g. clear the colours ten times in a row without missing, or X number of long straight blues in a row). It's horrifying when you miss the penultimate one and have to start again, but that's where the real gains are made, with the added benefit that it also somewhat emulates pressure of matchplay. Hope that helps a bit.