The old-fashioned advice about fluids and bed rest is true: your body does need rest to fight a cold, and fluids (water, tea, broth, juice) help thin the mucus so your body can get rid of it more easily.
There are also some over-the-counter products available that claim to help shorten the duration of a cold. Zicam claims to be clinically tested and proven to shorten the duration of a cold. Cold-Eeze lozenges make a similar claim. Both products contain zinc. Zicam comes in several forms, including a cotton swab which can be applied inside the nose. I suggest this form because the taste is slightly unpleasant. Both Cold-Eeze and Zicam, when taken orally, leave an odd, chalky texture in the mouth which I dislike. However, I dislike the cold more!
A pill supplement of calcium, magnesium and zinc, or as stated above just zinc. But I have heard not to take vitamin C and zinc at the same time because they cancel each other out or something to do with digestion...idk...need to do more research, but most of those cold remedies have both in them so idk
As people previously have stated, rest, NO SUGAR!, vitamin C, lots of water.. I personally have also found that dissolving sea salt into water, and then gargling with it is often useful. The theory behind this is that cold germs breed in your throat and nasal passages (so you can also squirt it up your nose if you choose).