Simple... I cheat. I'm lucky enough to have a LFS guy that will order stuff in for me, and actually perform a quarantine service. Yes, it's rare to find a LFS that is capable of, or willing to do so... it's also rare to find a customer that is willing to PAY for such a service. I am. I've seen folks go in and price check vs LiveAquaria in the LFS. Might as well stay home, there's no way a LFS is going to offer decent service, clean tanks, any form of quarantine at all, and still come anywhere near LA's prices. As I've said, trust is earned. My LFS guy has earned my trust.
I might also say this: It seems, sometimes, that folks are always adding fish to their tanks. I rarely add a fish, and never on impulse. I have 10 fish in my 140g display... and I'm not going to be adding any more, unless something happens to one of the ones I have now. Fish live a long, long time, if properly cared for. I strive to maintain a low stress environment for my fishes... I don't overcrowd, I don't buy known aggressive species, or species that I wouldn't expect to coexist peacefully with the fish I already have. I feed well (too well?? Perhaps) with high quality foods (LRS, Rod's, PE Mysis). I rarely loose a fish. Had a clown go carpet diving about 4 months ago... I might replace him, eventually. No rush. When I do, I'll select a clown that has been at my LFS for several weeks. I'll have visually inspected the individual as well as I can, I'll have seen him eating, over the course of a week or two, looked over his tankmates... everything I can do to make sure the fish is healthy before I take him home.
Once there, I'll acclimate, net him, and put him straight into the tank.
Could I end up, some day, having some sort of an outbreak? Sure. When/if that happens, I'll deal with it as best I can. Happily, so far so good. Haven't had a major problem in many years... last was an outbreak of GBD in the late 90's. Took out my entire seahorse herd.