So many good ideas in this thread! I definitely agree with:
1) "borrow" material and incorporate it into your own songs/practice as a way of learning the language more. Jazz and rock musicians do this by learning "standard" licks early on in all 12 keys and then incorporating it into their own music. It doesn't mean you regurgitate the licks 100% each time. Instead, after much practicing they begin to evolve into your own speech pattern. Many legends got their start from listening and then transcribing (be it just by playing along with by ear and/or actually using pencil and paper) the musicians that inspired them. They were teaching themselves the language of music.
2) Learn the rules. Then learn how and when to break them. Do what feels nature to YOU as a composer, while always striving to find your own touch or voice. At first glance these seems like a contrary statement but you have to first learn the words, phrases and overall language of music before you can really speak it. This doesn't have to happen with theory text books and formal classes but almost always happen with at least critical listening and the borrowing step from above.
Whenever I've felt like I'm running into a rut in my compositions I try a few things to stretch myself - take on a completely new style. Work with a completely new set of instruments/samples. Write in a key or mode that isn't normal for me. Write at drastically different tempos - really slow and really fast. Try different time signatures. Anything that can add a new element for experimentation and change. See how that changes what ideas you come up with.