I love adding a little hand-drawn or hand-painted something to my collage work. I think it adds an extra layer to the piece that makes the collage more visually interesting.
The Faber-Castell India Ink pens have worked best for me. They dry really fast and don't smear when I finally go to varnish my work.
I always varnish my work if it's something that probably won't be framed. Collages on canvas, cradled board, wood, and my sculptural pieces all get varnished.
Just make sure that your varnish is safe to use on paper. I use Mod Podge brand varnish, which is safe to use on paper, plastic and metal. You can find it at Walmart or craft stores.
Varnish comes in glossy and matte finishes. I personally like the gloss. I have found that one coat of varnish is all you need.
How do you add the finishing touches to your work?