It doesn't work very often. The key to getting it to work is that one of several factors have to be in place. First, if the mix and instrumental were mastered at the same time, it will always work perfectly. If the mix and instrumental had very similar mastering, it will mostly work. If the mix and instrumental were done at completely different times, with a completely different sound, it will never work. These days, it's almost a requirement in music (for released songs) to provide a mix and instrumental for anything of decent exposure. In something like dance music, where we are discussing this, most mastering is done by the person mixing the song. Therefore, you would have a high degree of success pulling out the vocal on anything done within the last couple of years, provided you can find the instrumental in the exact same mp3 compression as the mixed version. It would certainly be the case on anything I've mixed for dance music.
The big problem comes from the idea that I might mix something that's going to a real mastering engineer for CD/Vinyl. My instrumental (not always asked to provide this) would not be mastered in the same way as the full mix. Therefore, the results won't quite work or even not at all.
Here's the basic process, and you don't need a special program or EQ to do it. Get the mix, and an instrumental (the closer the mix and instrumental sound to each other all the way down to mp3 compression the more success you will have). Import them into whatever workstation you desire. Make sure they are perfectly synched with each other (I mean perfectly, the higher the amount of phasing, the more in synch you are). Then find a plug-in that allows you to flip the phase. The symbol for a phase flip switch is a 0 with a / through it.
In this photo, it's the button right above the output knob.
Once you've found a plug-in that allows you to flip the phase (most channel strip plug-ins have them), put that plug-in on either the mix or instrumental track. Click the flip phase button, and if you successfully did it, you should hear only the vocal (sometimes you hear a weird throbbing underneath, which usually isn't undesirable when putting music against something). Once you achieve the desired effect, if you can, record the acapella on to another set of tracks, import it into your session, and have fun.
Similarly, if you have an acapella and a mix track separate, but no instrumental, you can do the same thing to create an instrumental track. What you're doing is throwing the element you're trying to get rid of out of phase, causing complete cancellation of the sound you don't want. To create an acapella from a mix and instrumental, you are canceling out the music via a phase flip. If you need any clarifications, let me know.