Rabbut's seven rules of Discus keeping.
Note, these are not rules, just guidelines 
1) Keep them warm, 28-32 degrees centigrade is where they want to be. This will melt many potential tankmates, so choose companions with care. While looking at tankmates, you realy do need to be careful about what you mix with them, agressive, fast, nocturnal or large fish are a no-no with Discus. Plan your stocking arround the Discus if you want to have them
2) Over-filter through large and reliable canisters, or off a sump. I run at 7X an hour in mine, through two Tetratec EX1200's in a 335l tank. Many keepers say they do not like flow, but mine often cruse in the outlet at feeding time, realising that thats where I drop the food. They seem not to mind a current IME. You will also note that I am running two canisters, each of which is capable of running the tank alone. This is a redundant system, such that if one fails the tank does not crash, and it is done because Discus are expencive and you get very attached to them. A filter failure in a tank running one filter would put them at sevear risk
3) Clean water. The water needs to be clean. For adult fish, you are looking at 50% weekly waterchanges. I aim for twice weekly 50%'s, though juviniles need dailys as tenohfive says
4) Good food. Feed lots, 4 times a day minimum, of food in a 50-50 split between frozen/fresh and dryed prepaired food. Make the diet of good quality stuff, and keep it varied. Beware overfeeding though, as this pulls waterquality down
Mine get Vipakraft Discusin, Tropical quintette frozen and veg from Human meals as and when
5) Research, research and research some more. Many people consider Discus "hard to keep". This is mainly due to ignorance, advanced, mebe, but hard, no, not if you know the fish and keep up with care
When someone tells you they are hard to keep, hear "I was too ignorant to research properly and I blame the fish due to errors in my care as a result of lack of research" or "I am just repeating the words of another fishkeeper that doesn't have a clue about them either" as applicable for the person telling you. When you think you know enough to start dabbeling with them, try helping people with Discus on the forum. State you don't yet keep them, but that you would do X in Y situation. If the Discus keepers agree with you consistantly, great, you are ready. If you find yourself being corrected, thats OK, it just shows that you have avoided loosing one of your own fish to a potential mistake that you posted
No Discus keeper will follow your advice untill they have heared from a Discus keeper also, unless they are getting realy desporate, so you are unlikely to do any damage with any errors while checking your learning this way
6) Keep groups. They need a minimum group of 5, preferably more, be beware overstocking unless you love doing waterchanges though
7) Always keep them in a mature (6 month old +) tank. Since a new tank will force you to wait for them, focus on point 5 while you let the tank mature.
HTH
Rabbut
