I personally found the best way to learn was with flashcards, and also associating the kana with pictures. Thereafter, you just need to keep practicing (especially writing them out) and it will eventually stick. I'm just at the point now where reading and writing them feels natural and I almost don't have to think about which letter is which; I'd assume that is when you know you have become pretty much used to kana.
It'll take a short time to learn them all but will take a lot longer to get to a stage where you'll just be able to read them off like you would English.
Check our my resources page for learning kana.
Let's start off with my favourite method: picture association.
'Shi.' 'Shi has long hair.' From then on, everytime you see 'shi' you will probably be able to remember it's shape because you'll think of the long flowing hair. You can find loads of links for this type of learning.