MS provides "sp_getapplock", and the corresponding "sp_releaseapplock", to help you do this. With them, you can leverage SQL's already-built-and-tested locking mechanisms for your own logical locking. You also want to look at APPLOCK_TEST() (and maybe APPLOCK_MODE()).
SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".