You can either buy a pre-built sump or you can make a DIY sump.
They are pretty easy to make if you have an old tank laying around or maybe find one on craigslist. You will also need a way to pull the water from the tank to the sump. You can build a built-in overflow if you have an acrylic tank (preferably), or you can buy or DIY a overflow that hangs off of the back.
Basically what it does is starts a siphon and it pulls the water from your tank to sump below with gravity. The tank below has a series of filtration methods (your choice) and then there is a return pump that pumps water back into your tank. You can hide all of your equipment below the tank and it keeps the water level the same at all times. Also really easy for WC's.
I haven't personally built one yet, but I have been researching the crap out of them. I just picked up a 55G and I'm building a sump for it either this weekend or next weekend. I will make a work log of it while I do it and post it up here. I think it’s the way to go if you have a larger tank for many reasons.
#1. Keeps water level the same.
#2. Increases water volume.
#3. Easier WC’s.
#4. Hides all of your equipment.
#5. Localizes all of your gear in one place.
#6. Keeps the surface of the water clear of debris and other crapola that floats around for maximum lighting conditions.
- Marine Depot - Marine and Reef Aquarium Super Store
^^ those guys have tons of stuff for sumps, pretty $$$ though^^