Experiment with the trim control on the recording channel you're using. Gradually turn up the trim level while monitoring the pre-fader level meter. Try to get the level as high as you can without clipping the signal. This is the digital distortion that occurs with a peaking signal. It's decidedly unmusical and should be avoided at all costs.
Experiment with your technique on the microphone. If you're too far away from it when you sing, you will not get the most from it. If you're too close, you'll have issues with distortion.
Invest in an external pre-amp. The pre-amps in many mixing desks and digital interfaces do a good job, but upgrading to a higher quality external pre-amp can add substantial distortion-free gain to your signal and really bring your microphone to life. Some pre-amps offer a largely transparent boost to your signal, while others color the sound substantially. Which way to go is a matter of preference, and it pays to try out several models until you find one that suits you.
Add a little compression. Using a compressor on your signal will boost the level of the quieter-recorded passages while taming the peaks, adding a little spark to the sound of the microphone. When used sparingly, it can even out the sound of your recorded track and give you a consistent volume level. Be careful not to overdo the compression though; to do so is to risk losing the dynamics of your performance.
Take the foam windscreen cover off your microphone and instead use a mounted pop filter (see Resources). Foam screens tend to deaden the sound, while pop filters are more transparent and will help you control the hisses and pops from a voice without compromising the sensitivity of the microphone.