Unless facing a severe down hill putt or sideways breaker, always hit putts of 3 feet or less firm.
These 4 tips will help:
1. Take some or even all of the break out of a putt
2. Reduce the effect of small imperfections in green on your ball thus enabling it to maintain a straighter path (hopefully toward the hole).
3. Take away any chance of leaving the putt short or hanging on the lip.
4. Allow for a controlled stroke with less chance of pushing or pulling the putter off line.
I have found that one of the best ways to ensure that you strike the ball firmly is to do so towards the back wall of the hole. Try to make the ball fly across the opening of the hole and strike the back wall with some force.