One of the most common techniques to avoid this problem is using an extra sheet of paper.
Place the paper so it's under your drawing hand and on top of the part of your drawing you're not working on. Be careful not to move it around too much, using masking tape to stick it to your desk if necessary. If you use tracing paper, you'll still be able to see where you're headed through the sheet, instead of blocking off your view of the piece as a whole.
You can also start to build up your hand control. The smudging happens, generally, because your drawing hand is resting completely on the paper. On a horizontal desk, this in natural, but leads to smudging. Instead of resting your hand on its whole side, try using one of your pinky knuckles as a the only place your hand touches the page. This will minimize contact with the paper and reduce the chance of smearing.
If you're on an inclined desk or easel, you can change to an alternative drawing grip. The underhand grip, for instance, should keep your hand completely off the page.
Other than that, I'd suggest just being careful about watching your hands. Making sure your drawing hand isn't making contact with the paper where you've already drawn while you're drawing, and that your free hand isn't gripping the page on the penciled parts. Change your paper or hand angle as necessary to do this.
A note, pencils in the B range are softer and will smudge more and more obviously, so be extra careful with them. 4H and higher shouldn't smear very much at all, but they give you much lighter values.
If you meant the smudge after you finish them, I would recommend you purchase some fixative spray. It can be used for graphite, charcoal and pastel. Essentially, it adds a protective layer over the piece. There's also fixatives to use while working on them. I have no idea about these, but John Cavan does.
Aerosol hair spray does the same in a pinch, but will damage your work over the long term. However, if you're just going to scan it or take a picture of it, you may not care. The only time I've done it has been for keeping things from smudging long enough to get a digital copy for.