A clean mix is a difficult thing to achieve, and with reverb being such an important element of all modern music it is very important to treat it carefully to stop it muddying your mix. Here are a few tricks to help mix your reverb successfully..
Your choice of reverb and what instruments you use it on is the most important part of mixing reverb. This changes from genre to genre, song to song and with personal taste, but in general long reverbs will quickly ruin a mix if used excessively, while not using enough reverb can cause your track to sound lifeless. Don't rush your choice of reverb, and ignore preset names – use what sounds right. Personally, I tend to use quick, bright reverbs for snares, and long and lush but fairly quiet reverbs for vocals.
Instruments in a mix are often passed through high-pass filters to remove unwanted frequencies muddying the bottom end, but the reverb is often ignored – this can be problematic, especially if some instruments with reverb take up a wide frequency range (a piano, for instance). Adding a high-pass filter to the reverb channel, with a cut-off frequency between 50-500Hz or higher still, quickly stops long, lush reverbs from swamping the bottom end in a wave of noise.
In the same vein, reverb can also benefit from being put through a low-pass filter. Often instruments which sit high in the frequency range have reverb added to them, such as vocals and cymbals, but too much can cause a constant 'hiss' or 'fizz' which hides the instruments behind it. If a long reverb is desired then it can benefit greatly from the higher frequencies being removed to clean up the top end. Most reverb plugins will, by default, have high-frequency dampening, but this does not work in the same way – higher dampening causes higher frequencies to die away faster, rather than omitting them in the first place. Experiment with the cut-off frequency and dampening amount to find what suits your track!
Something to note is that a band-pass filter with a large width can have the same effect as a high-pass filter followed by a low-pass filter, with the added bonus of using up less CPU.
Do you want lots of reverb, but can't stop it clouding over your instrument and hiding its details? A simple trick is to add a side-chain compressor after your reverb effect – route the instrument as the signal to cause the compressor to activate, and lower the threshold to your liking. Experiment with the ratio, and if the instrument in question has quick dynamic changes (eg. vocals) then use a quick attack and release. This will cause the reverb to be compressed when the instrument is playing, and fill in the gaps when it's not; without changing the reverb itself, your instrument will now be much cleaner and easier to hear!
Many genres of music - Electronic Dance Music (EDM) in particular – contain a strong beat which needs to cut through the mix. Side-chaining the kick drum to a compressor on your reverb can help clear the mix for the beat to punch through, and can also create nice movement in the reverb as it swells to the beat.
Gated reverb has been around since the 1980's, and is still incredibly useful. You don't have to use it in such an extreme way as it once was though - try adding a gate after your reverb effect to stop the tail end from muddying up your mix once the main body of the reverb has died away. You may be surprised how much seemingly-harmless quiet noises can build up and swamp the background of your mix.