I've printed out a photo, put it in a sheet protector and doodled with a dry erase pen. Get a piece of heavy clear tablecloth vinyl, lay it on your quilt top and draw with wet erase pens. (Just be sure not to draw off the edge of the vinyl!! Put some blue painter's tape around the edges so you know where to stop.)
Or if you want a higher tech method, you can take a photo into PhotoShop or another such program and doodle on that photo (works best if you have a tablet to draw with).
As far as how to decide, I often let the fabric guide me. If it's a romantic, floral type fabric, I'll use a complementary floral or leafy design. If the quilt is for a child and is fun and whimsical I might put bubbles or stars, etc.
I do have a few photos on my website to give you an idea of what I'm talking about
:
http://www.andicraftsquilting.com/quilting-gallery.htm
The more you look at finished quilts, the easier it will become. There are some good books on the subject too. Good luck!