I’ve always advocated that Mac admins join the Mac Developer Program in order to get early access to OS X builds for testing and deployment planning.
I still think that’s a good idea. But if for whatever reason you can’t, Apple has a new program of interest:
I think it’s unlikely this will get you access to early builds of 10.10 (or whatever it’s numbered), but you can test 10.9.3…
Managing OS X 