Sibilance is the term for harsh "sss" or "sh" sounds in a vocal recording. There are a few ways of dealing with the problem:
Before recording: Look for microphones with a "flat" response, which tend to suppress sibilance. Many high-end vocal mics deal with the problem with larger diaphragms and other built-in components that moderate these specific signals. You can also run the microphone through signal processor that features a "de-esser," which you can adjust to cut sibilance.
During recording: Experiment with the microphone's position relative to the speaker/singer. Often backing the microphone away a bit, angling it down, or having the person sing slightly to one side of it will reduce the sound energy that's causing the sibilance to begin with.
After recording: If your vocal track features too much sibilance, you can use sound editing software to reduce it. Be careful, though - just cutting the sibilance frequency (around 8 kHz) across the board can deaden the sound of the track. For optimal results, identify the specific areas on the waveform which sibilance is a problem and selective cut the 8 kHz volume in those areas.
Remember, too, that your sibilance problem will be different with each performer. There's no "one-size-fits-all" solution for every recording situation.