How to protect old paper when used in a collage?

I am gathering paper from various sources (auction catalogs, every day magazines, newspaers, old textbooks, etc.) and would like the end product to remain stable, that is, not to change too much (yellowing, fading, becoming brittle, etc.) As an adhesive I will use Liquitex Matte Medium. Is there anything I can do to make a collage long lasting? I would like that the final product is finish-wise art gallery ready.