Apply a little pressure during your practice. For example, if you're practicing five-foot putts, the following might be an example:
Make < 40%: clean out the garage
Make 40 - 60%: good job
Make > 60%: treat yourself to a nice big steak and a beer
The net time, maybe you bump the percentages to 45/65. Find the point at which rewards are truly rewarding - and difficult to achieve. As with the regular game of golf, you are your own referee. Be honest or your game won't improve as it should.