Whole books have been written about just this, but a few tricks to play with:
1. Levels. When mixing vocals with backing tracks, very minor nudges in the relative levels of the vocals vs. the music can make a big difference. Often you have to get down to word by word or even syllable by syllable adjustments to make things sound just right.
2. Compression. Since you've downloaded pre-mixed backing tracks, there's a good bet that they've already had more compression applied than would be ideal. You may find you need to match this (i.e. make things over-compressed when you listen to the vocals solo) to make them match the backing tracks.
3. Reverb. You mention that you've already added reverb to your vocals but, assuming the backing tracks also have some reverb (be it electronic or just the general room ambience from where it was recorded) you have to make your vocals and the music sound like they were recorded in the same space. Since you can't change the backing tracks, this means listening to them and doing something similar to the vocals even if that's not exactly what you want when you consider the vox alone.
4. EQ. To some extent, you have to cut a hole in the backing tracks to leave room for the voice. If you had access to all the individual stems from the music, you could get creative with panning, lots of individual EQ, etc. etc. However, with a pre-mixed backing track I find I get best results with a bit of a blunt instrument. Use EQ to slightly cut the main vocal range (probably something like 200-800 Hz but that depends on your vocals) out of the backing track. It's doesn't take a lot of cut--probably 3-4dB with a slope to make the change gradual--but this can really help the vocals sit in the mix.
The reality is, you'll probably just need to play with all of the above. One other trick I use (and maybe this is just me being weird) is that, when I'm getting close, I set back from all the controls, close my eyes, and concentrate on the music rather than displays and buttons. This often helps me hear the last bits I want to tweak. Similarly, a short coffee break to "freshen the ears" can help a lot too.
Hope some of this helps,
Bob
