Too much low-end rumble on stage : If you're sound system is like most rigs out there and you have a pair of mid-hi's on top of a couple subs this trick might help you out. If it seems like you have more low-end on stage than you do on the dance floor you might have a standing wave problem. Try reversing the polarity on one of your sub cabinets. This is very easy to do if you are using banana plug connectors going into your amp. Pull one of your banana plug connectors out and just turn it around so the gnd notch on the connector is on the red post instead of the black post. Remember to do this for only one cabinet. (Unless you have multiple subs on each side of the stage, then you will have to reverse the polarity on that whole side.)
If you use a connector other than banana plugs you might have to get a little creative and make a small pigtail adapter with the proper connectors for your situation, just as long as you reverse the polarity on one side. You might also check your crossover for any kind of phase reversal switches on the low outputs.
The end result should be less low-end on stage simply because you moved the standing wave to a differant location (ideally on the dance floor).
Graphic EQing Tip
Try this the next time you reach for an EQ slider. Instead of trying to add the frequencies you like to hear such as more low end to punch up the kick drum, try concentrating on the frequencies you don't like and taking them away. If there always seems to be a harshness in the vocals try finding the one or two frequencies that are causing the problem (possibly 2.5k - 4k) and backing them off. Also if things sound kind of hollow in the mids try finding the offending frequency (400 - 800) and reducing it. Many times the problem frequencies are the resonant frequencies of that particular room and by reducing them you're decreasing their energy levels. After training yourself to recognize some of the most common problem frequencies and reducing them you'll be surprised at how the entire sound of your system will be cleaned up. (Get a 31 band EQ, you'll be glad you have the ability to hone in on those specific frequencies.) For practice, the next time you set up your system, make sure no mics are plugged in and play some good sounding recorded music through the system just loud enough to hear it clearly, no louder. Then adjust each frequency on your EQ, one at a time to its extents, all the way up then all the way down. You should clearly hear what each frequency sounds like. As you go through each frequency make note of which ones that are really unpleasant. After a while you'll begin to recognize problem frequencies at the gig and be able to knock them back smoothing out the entire sound of your system.