This video describes cleaning the water supply holes under the toilet with a piece of wire. It's a good idea, first, to clean the underside of the toilet rim with a toilet brush. In areas that have hard water, the build up of mineral deposits will occur in these supply holes. It's good to clean them out on a yearly basis.

Another common problem for a weak flush is not allowing the flapper enough space to open all the way. You just need to shorten the chain from the flapper to the flush handle.