One of my favourite things to do to figure out a design that will work for *this* quilt, is to use a piece of clear plastic tablecloth (bound in blue painters tape) and start playing with washable markers. Try a few designs, see if you like the look of the design with that quilt area. My design plastic is about 18" x 24". Wipe off the marker and try different designs. This also gets you a little into the groove of how to move around in the design - builds muscle memory and also gives you a preview of where you might quilt yourself into a corner.
There are absolutely *no* rules
I've found that I pay a lot more attention to the quilting designs in magazines now - as well as the quilt pattern and fabric, the finished quilts have interesting quilting.
Also, when I started quilting, I used decorative stitches quite a bit that served as the quilting.
One of the ways to find inspiration is to visit some of the blogs of QuiltingBoard members whose quilting you admire.
I've been enjoying the sketch book inspiration at 'May Your Bobbin Always Be Full' blog
http://luannkessi.blogspot.ca/search.../Sketch%20Book . Often you'll find more excellent inspiration in the blogs that the bloggers follow ...
You can practice, practice, practice quilting designs on smaller items such as table runners, placemats, wall hangings, purses, jackets, hot pads, to build courage for bed quilts
Enjoy the journey!