Hi,
Discus arent really dificult fish that get sick all the time, its just that they dont like surprises. They mostly do well where everything is kept constant ie temp (84-86F), PH, hardness etc. They prefer low lighting and the brown stained water you get from having bogwood or peat filtered water, if your water is hard, with a ph over 7.5 your best bet is to get and RO unit to soften the water and bring the PH to a more comfortable 6-7. The RO will also remove chemicals like chlorine and pesticides from the water.
The water must have no ammonia or nitirite at all and should be up and running for 3 months or more.
If you can give them constant conditions like these then they are easy to keep, if your tank is having wild PH changes or is full of aggressive and over active fish like tiger barbs then the Discus will never settle in and will get sick.
Always check what PH and hardness discus are being kept in before you buy, some local shops here keep them at PH 5.5 and some at PH 7.2, so obviously the fish from the second shop will settle in better if your PH is nearer this mark. If you have only one source, try to match their water and keep it stable like this for at least a month.
It is a myth that these fish are more prone to diseases than other fish, it is just harder for someone with no expierience to give them the conditions they need.
Of course there are exceptions to this and people will say they bought discus and dumped them in their raw alkaline tap water with a PH of 8.5 and had fry, but the points above will generally improve your success rate.
Ken