Without buying something else? The best thing you can do is use both. That's what I do since I don't have any monitors either. You need to use them as references. Hell, throw another set of something crappy in there. It'll never be as good as if you had actual monitors but at least you'll be halfway there. Here's what I do...
1. Use your computer speakers and set a rough mix.
2. Pop in your headphones and reference the mix. Tweak if necessary.
3. See how it sounds with the speakers again.
4. If you have anything else to plug in and try, do it at this time, I use my iPod headphones. Tweak if necessary.
5. See how it sounds on the speakers again.
6. Bounce, and see how it sounds in your car. I usually bounce the dry source files to see if my editing/mixing was effective or taking a step in the wrong direction.