Ensure that all cables are properly sized. If you're standing a few feet from your tube preamp, avoid using a lengthy foot cable. If you have cables coiled up or, worse yet, crossing other active cables, you'll be getting some unwanted feedback as sound is transmitted through the cables. Purchasing well-insulated cables helps with this problem, but even the best insulation cannot fully shield your cables---always make sure your cables run in a straight beeline from your instrument to the preamp.
Keep volume and gain at moderate levels. This external fix is easy, yet many musicians overlook it. It can be tempting to crank the sound on your tube preamp, but keep in mind that this can cause earlier tube burnout and unwanted distortion. It's better to play it quiet and clean than loud and distorted. If you consistently require maximum volume, consider replacing your old tube preamp with a higher wattage model.
Do not change cables while plugged into the preamp. Changing cable configurations can cause loud "pops" of sound that are harmful to your preamp, creating wear and tear that leads to distortion.
Keep instruments at a distance. Spreading out your bassist, guitarist and drummer is a good way to reduce unwanted distortion. If you've got active instruments too close together you'll get noisy preamp feedback.
Use a sound hole cover for acoustic-electric instruments plugged into the tube preamp---these are also called lute hole covers. Sound hole covers help prevent feedback and distortion.