I find that sometimes the quilt will tell me how to quilt it......I know, that sounds weird, but the quilt pattern or design in the fabric will often give me an idea how to quilt. For instance, if I have a bright, fun, funky little girl's quilt, I'm not going to put a traditional pattern like clamshells or baptist fans on it, I'm going to use something like funky flowers. But I won't put those funky flowers on a quilt made from civil war reproduction fabrics - then I'd use the clamshells or baptist fans.
So first look at the fabrics used, and think about who the quilt is going to be for, since you probably don't want hearts or flowers on a quilt for a man. And decide if you want to do custom quilting or an overall design. If the fabric is busy and the quilt is meant to be dragged around and used a lot, an overall design will probably work fine. If there are lots of open spaces of plainer fabrics, or lots of applique, then custom might be the way to go.
The other thing that has helped me tremendously in knowing what to put on a client's quilt, is studying other quilts at quilt shows, in books, and online. I spend a lot of time looking at the QUILTING, not just the piecing or colors, to find what works and what doesn't work. And as someone else said, there is no right and wrong, it's a matter of individual preference. Some people love feathers, others don't really care for them, some love dense quilting, others think it's too much.
If you can post a photo, maybe we can help you with some suggestions.