Now that we've lived here for well over four years our knowledge of French has increased to a level that we once never thought would be possible. During our first couple of years we went down the traditional route of going to classes and reading lots of grammar and verb books. These days we watch a bit of French tv, read the papers regularly, speak often to our neighbours and dip in and out of various rather light-hearted how to books and it just seems to come much more readily.
There is also the tried and trusted method of just saying a particular English word but with a French accent or taking a guess at what a word should be. It's amazing how often that works but now and then one can slip up. For example the word deadline in French is delai ; the verb to stay is rester; the English word sensible is sensitive in French.
I've met some expats who have been here for about 15 years and yet they can still barely pass the time of day in Fench. I've got no time for this because I think if you are going to live in a country then you've got to speak the language.