How can I improve my French without actually trying?

Sadly, we can't all go to France on holiday to polish up our French...

A way of learning vocabulary that I have found very useful and also enjoyable is to listen to songs and the radio in French. You could also watch films or TV series (Les Revenants is a good one!) with subtitles (either in English or in French depending on how confident you're feeling). Plus, listening to French speakers will help improve your accent!
 

Another tip is to change your phone or your computer's language settings to French; you might get a bit confused at first but soon enough you will get used to it and have learned lots of new words.