I bought a nice dog harness manufactured by a company called "Ruffwear" (For Dogs on the Go).
The harness style I bought is really designed for walking, but it includes some nicely padded chest and stomach straps and it has a handle in the back, designed to lift the dog with, that you can thread a seat belt through. The harness is also designed to hold doggie saddlebags.
It was the best fitting harness I could find for my particular dog but it wasn't cheap and I plan to use it with a seat belt when she's riding in my Jeep with me. I'd recommend it.
http://www.ruffwear.com/Web-Master-Harness?sc=2&category=1131