Reserving your Guild Wars character name for use in Guild Wars 2 is easy: just log in to Guild Wars. Any "active" Guild Wars player -- meaning you've logged in since Jan 1, 2012 -- will have his or her name added to a list which will be active during GW2's Headstart Access period and launch day. During those periods, new players will not be able to claim your name.
The reserved list will be opened to the public sometime after launch day, so if you don't claim your name early, it'll be up for grabs. Also note that the list of reserved names will be generated before the Guild Wars 2 launch, so if you haven't logged in this year, make sure you do it soon to ensure that your name isn't left off. You wouldn't want to be stuck with "Wizardex," would you?
The full explanation of how name transfers will work is available in a post on
the ArenaNet blog