Eliminate any nontechnical source of hiss in the recording space. Check for noisy ceiling fans, refrigerators and air-conditioning units and turn them off during recording. Removing the source of hiss before the recording begins is always easier than trying to fix the problem later.
Plug your microphone into an external pre-amp and then connect the pre-amp directly to the recorder input. Unless you have a high-quality sound card installed in your computer, plugging directly into the mic input of your computer will generate unwanted noise.
Experiment with the gain control on the pre-amp to get the optimum input level. Try to get the hottest signal possible without clipping the signal. If the LED indicator light flashes, the signal is clipping, which means digital information is being lost. In this case, gradually reduce the gain until the flashing stops. Don't reduce it too much, because when you raise the recorded track in order to be heard, you'll be raising all ambient noise too, which will result in more audible hiss.
Move the mic away from your computer and place it on a floor stand. Computers can be noisy, and if your mic is sitting on or close to the computer desk, it will often pick up and magnify the hissing noise of the computer's fan.
Adjust the height of the mic stand so that the mic is directly in front of the singer's mouth, at a distance of between 6 and 12 inches. Rotate the mic until it's slightly off-axis, so that the singer's voice isn't directly hitting the diaphragm of the mic. This will reduce sibilance and give you a cleaner recording without high-end hiss.
Attach a pop screen to the mic stand. As well as helping you deliver a smooth vocal level by keeping your mouth a consistent distance from the mic, this circular mesh screen will also lessen the effect of plosive sounds (pops), as well as reducing sibilant hiss.
Use the equalization (EQ) controls in your recording software to tame the high frequencies where hiss resides. Cutting all high frequencies would certainly clean up any hiss, but it would also take the life out of your vocal sound, so a light touch and a little experimentation is required. The high frequencies found in the 16k-20k range don't affect the sound too much and will only give the recording a little "air," so removing these should help alleviate the problem.
Use a noise gate to treat your vocal track. Whether a hardware device or a software plug-in, a noise gate will eliminate any noise between vocal phrases. The hiss may still be there on the vocal track, but since hiss is most obvious in the quiet spots between phrases, the gate's muting of the track will render the hiss largely unnoticeable.