Not sure what kind of software you're using for mixing, and how many plug-in type features it has. If it has a multiband compressor with side-chain, you might try throwing it onto the instrumental mix, using the vocal track to key the side chain, and compressing a band around 500-600Hz. This may open up space for the vocal by scooping out a little of those competing frequencies on the instrumental track only when the vocal is actually doing something.

Ditto what L-dog said. I deal with this a lot in hip hop, and with remixes, and mutliband compressor is a life saver. If you need a little more extreme of a pocket for your vocals, try putting a compressor on the 2 track mix & keying the input of the compression from the vocal track(s). (I'd love to hear some underground hiphop start doing this like Aesop Rock, Anticon, Illogic, etc) A lot of the sub-lows get in the way of the music track, so I usually roll off up to 200 htz on the vocal rap track.

Hey I am going to go across the grain here and say don't use the multiband compressor, especially if you have no experience with one. They can be tricky to setup right and can ruin the mix than enhance it. A lot of mastering guys won't even touch them. Again, if you are recording the vocals, than that's where the right mic/pre/Eq and comp is crucial. I would say try to get the right balance of them all when tracking and when you mix it will be easier to tuck the vocal. Some hints: Not only use delay on the vocals, but a slight reverb on the delay, as well as on everything. Ride the reverb or sends, so when they hit the reverb it won't be too much. Use an overall comp on everything, a transparent one, which can glue everything a little tighter. Setup two different vocal tracks, with different comps and EQ settings, if there are different sections in the tracks. One vocal can be the verse(usually simpler instrumentation), the other the chorus(usually more dense an agressive). Hope some of this stuff can help.