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.

Sounds good, that's pretty much what I do anyways since I like listening to my own music... lol. It still sounds nothing like the pros though... but I guess as long as it's a coherent mix people won't mind, right?

I'll try using my speakers more like you said. Thanks.

"but I guess as long as it's a coherent mix people won't mind, right?"

So long as it holds together, but you'll find it easier if you can hear where the muddy areas are. That can only be achieved by higher quality setups.

here is what i suggest: pop in your favorite commercial cd in the style of your project and use it as reference. keep switching between your mix and the commercial one to get closer to a satisfactory result.

After i mix on my headphones i listen to it in my car. 311, does that too!