The art of mixing is controlling space really. It's a bit like putting furniture in your house. You could just bung everything in the middle of one room, but it won't be very practical or aesthetically pleasing. But moving the items around, putting everything in it's own space, and not trying to put two items in the same place will make the room seem much better.

You have at your disposal the volume levels of the parts, eq, panning, effects (reverb, compression etc). EQ and panning in particular can be very useful in separating parts, as can reverb (try using it as "ambience" rather than a big reverb effect).

Try keeping the vocal and bass heavy instruments in the center, and leave a hole for them with the other instruments by arrange them to be slightly out of the center. Processors like the Ultrafex, Vitaliser or Waves S1 can also help here by widening the stereo width, in effect pushing everything out a bit wider, which leaves a bigger hole for the vocal.

Also, just because a part is recorded doesn't mean it necessarily has to be loud in the mix. Often being subtle results in more spacious mixes.

Try using automation to create more fluid, "breathing" mixes rather than static ones. Try pushing the master level up a snadge on the choruses for more energy, and bringing it down for the verses.

Also, the song arrangement will form a big part of what the resultant mix sounds like. If you arrangement is clumsy, has competing parts and is too busy, it will be quite difficult to mix as you have to work harder to separate the mix elements. So be thinking about mixing the track even as you are writing and recording the song - sympathetic arrranging will go a long way to improving your mixes.

Lastly, listen to other artist's music which has a similar style and composition to your own. See how they have done things (and more importantly why). Copy a few of their ideas and try them out.

And turn the vocal up.

What's the most important thing in the mix? The vocal. Build everything around it, don't build everything first and then add the vocal last. If the vocal sucks (sorry, these damn Americanisms just get into your brain if you hang around the net long enough) - If the vocal doesn't sound very good, this will make the mix sound poor however hard you work. But if you have a great vocal, it will lift the mix instantly, and make it (psychologically) sound much better. Humans like voices, our ears and brains are attuned to them more than any other sound.