vortex has a point, the easiest way is to be in contact with native French speakers every day. That's the way I learnt. I went to stay in the south of France when I was on holiday from Uni and did an exchange the following year.
It not only helps with the vocabulary, but it also teaches you the correct inflections and the natural way of communicating.
I tried using one of those CD language books and found that the only use for them was to help me concentrate on my script and storywriting!
Get a french step-grandmother, mine's german, yeah, I know German now, but I'm much more familiar with the vulgar phrases that have been used towards other family members!