Putt 3 golf balls from 2 feet and if you hole all three, go back to 3 feet and try from there and so on. If you miss one go 1 foot closer to the hole for the next 3 balls. If you miss 2 or more go back to the beginning.
Similar process to the ladder, but this time there are four balls around a hole at the four points of the compass, north, south, east and west. This drill is better if the hole is on a slope so that you get a putt that turns each way. Place the first circle at 2 feet and move out from there if you hole them all.
Play matchplay with a friend on the putting green. Nothing like a bit of head to head pressure to sharpen up your putting!
Take two balls and play round the putting green in no more than 2 putts per hole. Once you have done this in one direction, reverse the direction of the round and do it again!
Practice aiming at something smaller than a normal hole. Try another ball, a tee peg, or even the leg of a chair on the carpet at home!
Spend most of your time practising putts within 6 feet of the hole as this is the length you will need to hole most often.