1) Get Really Friendly with Your Neighborhood Copy Center:
Color copies of old journal pages can be cut into strips to make borders on new pages. Color copy bits and pieces make excellent backgrounds for small art such as ATCs, inchies, and postcards. You get the look of complex layering without doing all the work twice. Have a painted canvas that didn't quite work? Cut that canvas off the frame and get thee to the nearest copier to recycle that painting into cool background material.
2) Get a New Perspective Through Your Camera's Viewfinder:
I take pictures all the time of my work, not only to build my portfolio, but to see that work in new ways. Take close-ups of small doodles, important focal images, or interesting background bits and use those detail photos to make new art.
3) Dance Between Mediums:
Look at symbols that repeatedly occur in your work. Let those lexicon images become inspiration for hand-carved stamps and use those stamps in your backgrounds or as focal images in new projects. For example, I recently painted a fox image on a journal page that I loved so I drew that image again on a smaller scale and created a stamp. I plan on making a series of greeting cards with my new fox stamp.
Recycling your art is a great way to get more mileage out of all your hard work and as long as your work doesn't contain copyrighted material in the first place, it is a fantastic technique for building a library of collage fodder that is uniquely your own.