For years, I've used the same method for reviving my strings when they start to lose their luster, particularly the wound ones.
1. Loosen the low E string.
2. Take an old T-shirt, wrap it around the string and wipe it vigorously up and down the neck until the string starts to heat up under the cloth. (Don't start a fire!)
3. Tighten the string back up to pitch and repeat steps with the remaining strings.
This really does revive the strings and bring back the sparkle. For just a quick wipe-down cleaning, I use a Tibet Almond Stick (available at any furniture or hardware store for about five dollars), followed by a wipe with a clean cloth. The Tibet Almond Stick is great for covering up small scratches too!