Bob's 27 is a great little practice routine I've been using for the last few weeks. It was devised by Bob Anderson and concentrates on Doubles accuracy.
You can keep a note of the running score on paper, in your head, or if you have a laptop handy DartPro handles it nicely with it's usual stats and graphs to help you identify the doubles you have the most trouble with.
You start off with 27 points and go round the board throwing three darts at each double from 1 to 20 in sequence, finishing on the Bull if you prefer. Every double you hit earns you the double's score and is added to your total, but missing a double with all three darts means you must subtract a single double from the total. Once you're down to zero or less the game is over.
I've only ever been right to the end of the game once, but for me it's a good bit of practice on the lower value doubles. That shot on D7 (pictured) is my best ever shot in Bob's 27 - I've not managed three in a given double before or since.