Creativity is a muscle that must be exercised regularly to stop it becoming weak. If you want it to be there when you need it, you can never loose it, once your creativity is gone past a certain point it can be very difficult to get it back at all.
The most obvious way to exercise your creativity is to use it as often as possible, this is not always easy though. One thing, which I use, is to think up two or three different emotions and imagine a person feeling that emotion. Then imagine they are on a film and write music to go in the background. Emotions, which work well in this exercise, are: happiness, sadness and fear. These are the easiest emotions, however they are not always the best to use. If you want to exercise a muscle, you don't lift 3 kg weights all day or you will get nowhere. I suggest trying harder emotions to write for such as anger, loss or self-pity. If you are feeling really good, you could try and mix some of the above emotions: your character could be angry due to loss or afraid of someone else's anger. The most important thing is that you don't use the same emotions or combinations of emotions every time you do this. You need to be able to write well to make your audience feel whatever you want them to. Music is a very powerful tool, which can make people feel anything and you need to be able to manipulate this every possible way.
Another good way to keep creativity alive is to go on a lyrics website, choose a song by a completely random band who you have never even heard of before and then write music to go with the lyrics. Think about what the song is about, how the lyrics go together in terms of rhyme and rhythm and how you want people to feel when they hear those lyrics over the music. Bare music can be a powerful tool, however, with lyrics over it, it can become even more powerful. If the lyrics of a song are close to you, the song hits you more. This can be done in reverse to keep your lyrical muscle exercised. Find an instrumental song and put a vocal melody and lyrics to it.
Something I once read somewhere on the same subject is to put the T.V on mute. Watch the screen and improvise the theme tune. This not only exercises your creativity but also improves your improvisation and ability to think on your feet. If you are finding it hard to actually play the music along to the images, just imagine the theme tune (i.e the music you would put to the images you are seeing). This exercises your creativity just as much as playing the music but will not be as beneficial to your playing.
You can use songs, which have already been written to exercise your own creativity. It may sound crazy but here is what I mean. You can do one of 3 things, number one: listen to half of a song, then turn it off and write the rest. Make sure you keep it in the same sort of feel as the song. The next one is almost the same except this time you listen to the entire song and then writing another song in the same style. Again, thinking about feeling, the whole point of music is to provoke feelings inside someone so this is the most important thing to think about when you write music. The final, and hardest, way is to learn a song and by changing the rhythms, tunings and everything except notes, try to make it sound different. Try and make a major song sound minor by putting a slow rhythm to it, or the other way round by putting a bouncy rhythm to it.
Well, that's all guys. I hope I've helped you keep your creativity easier and in a fun way. Peace out :-)