I struggled with this for a long time -- I used to just buy anything that I thought was interesting or colorful. But then I was always worried about territory wars (my aquarium is only 30G), and it didn't look very cohesive.
I switched to about 90% Ricordea looking for all types of morphs, and suddenly I actually like the way my tank looks. It still is so hard to resist new things though. I guess a second aquarium might be in order.
To me, this is the hard part -- a tank with a few large corals looks more realistic than the barage of everything imaginable, but the temptation to buy more types is just too strong....