So far I've never intentionally gotten rid of any - I just keep adding to the collection! The only ones I've lost are those that died - a mature male Aphonopelma that I'd caught as a sub-adult, a mature male P. vittata that I'd raised from a sling - and a juvie P. irminia that lost a leg in a bad molt and was finished off by the damned phorid flies.
But I have actually been contemplating that very question as I've begun running out of room to put things - and I still keep seeing things that I want to get! I do have a few cages that I expect to have vacant soon (another mature male Aphonopelma who hooked out almost a year ago and a female solifugid that I'm contemplating releasing, just because they don't do well in long-term captivity) but once those are filled, I'll have to start making the tough decisions. I would like to get another P.irminia or maybe another Avic of some sort. There are also several other species that I like the look of (though some are a bit too "pet-hole-ish" for my taste) - but in order to get them, I'd need to get rid of a few things.
The ones I give preference to are (in order of importance): those I've raised from slings (they're my babies - I would never get rid of them!), those that are calm enough to handle (*gasp* Blasphemy! Prepare the angry mob!), those that are particularly big or especially aggressive feeders (because I use them in classroom presentations and the kids LOVE watching them pounce on their food almost before it hits the ground! They are also always impressed with the T. stirmi for sheer size, even if she mostly just sits there.), and those that are especially pretty. The ones who are at the top of my "to go" list if I ever to get rid of any are the L. parahybana and the Grammostola sp. Concepcion or sp. Maule. (I like both of the two Grammos - but they're practically two of the same spider for all intents and purposes.)