One of the many reasons why I like turning to the art supply company called Daniel Smith is because their published catalogs always contain great articles and technique tips for various kinds of mediums. They also have many articles available on line (their on line instruction site).
Here are some various articles on working with collage as a medium:
"Painting Outside the Box" by Pat Weaver.
Or, how about a watercolor stained paper technique demonstrated by Brenda Swenson?
Explore other possibilities with artist Dixie Parker-Fairbanks.
Sandra Duran-Wilson lives up in Santa Fe is a mixed media collage artist. I love visiting her blog and she sometimes does demos for the Artisan Art Supply Store. You can also take classes at her studio in Santa Fe. Her background is very extensive, so if you want a lot of 'bang for the buck' so to speak, consider taking a class or scheduling a class with her.
I could go on and on with the list of sources for exploring collage as a medium. But, I won't! Aren't you glad??