Find a tutor. Working one on one with a skilled teacher is the best way to quickly learn any subject.
Use repetition. Every time you learn a concept, do several problems to reinforce that concept. For example, if you learn how to add fractions with common denominators, have your tutor or another adult come up with several problems involving adding common denominators.
Don't repeat too much. Doing five pages of dittos is probably a waste of time. Do enough problems to get the concept and drill it into your head a bit, but don't keep at it until your eyes glaze over.
Work on math every day. Setting aside a block of time every day to study math will help to keep the concepts fresh in your head, meaning that you have to spend less time reviewing.
Begin each day by reviewing a few recent lessons briefly. One of the biggest problems people run into when trying to quickly learn math is forgetting previous lessons and having to review basic concepts. By spending a few minutes every day reviewing, you can make sure that the mathematics you learn goes into your longterm memory rather than being forgotten after a couple days.
Use flash cards, mnemonic devices and other memory aids. Using flash cards is a great way to memorize your times tables, while mnemonic devices are a good trick for more complex concepts.