An important task that greatly depends on how you want to go about it; with crying (quick but painful) or without crying (will take longer but more gentle) or somewhere in between.
In desparation and having read lots of different books, I found one that summarises almost all of the current methods and it has a great online support forum where you can post your specfiic situation and get help/advice from the site's 'team'. When you join online you get an immediate electronic download of the book which is shipped in hardcopy a few days later. www.babysleepanswers.co.uk
We went for short and painful i.e. controlled crying - very sucessful and we now have a champion sleeper, but a bit traumatic and not for everyone. If you are not 100% committed to it or think you might not cope, I would stick with a gentle method that works over time. I have friends that swear by the Elizabeth Pantley 'no cry sleep solution'