Identify the source of the hum. Turn off, unplug and remove each piece of gear one at a time. If the hum appears only when one certain piece of equipment is incorporated into your signal path, you will know where the hum is coming from.
Run all of your gear through an AC power conditioner. Relatively inexpensive and quality power conditioners are manufactured by Furman. A power conditioner regulates the power coming in to your equipment, protects your equipment from spikes in current and can help eliminate noise coming through your system. Power conditioners usually start with eight different outlets so this should be able to power all of your gear.
Run all of your equipment on the same circuit. It may be hard to figure this out if you are at a venue and you do not have the luxury of turning off circuit breakers to identify which circuit is where. If you are at home this is an easy thing to do. A good way to make sure you are not running equipment off of different circuits or different phases of power is to just run everything off of the same outlet if you can.
Remove gear from an environment that can cause noise. Make sure that no metal gear is resting on a metal table without rubber feet or add something to separate the surfaces.
Straighten all cable. Do not use scrunched, tangled or bent cable. Run power cable and audio cable separate from one another. If you have to run power cable and audio cable over each other run them perpendicular to each other at a 90-degree angle.
Ground your turntables. Turntables have an attached wire with a U-shaped metal hook on the end that needs to be grounded. Attach it to a screw on your DJ mixer.