We have a new plugin called GriefPreventionFlags. Please see the GriefPrevention Tutorial Page for more information about GriefPreventionFlags. You can set entry/exit messages for your claims, and maybe in future (not yet though) even turn pvp on inside them. It’s worth having a look at the available flags. Also it’s a premium plugin so not every server has it. ;D
Also, on the Ranks -> Admins Page there are all possible flags listed (for admins only).