Experience is needed to unlock new Characters. It is awarded on the death or level transition screen, and depends on the days survived. Each survived day adds 20 XP. Subsequent games add to the existing experience (Cumulative).
More than one character can be unlocked at once. For instance, if a player only has Wilson unlocked but earns enough experience to unlock Wolfgang, both Wolfgang and Willow will be unlocked.
(Turning off Reign of Giants on the start menu, you can choose different characters here - Makes surviving a lot easier as it rains less)
Hints and tips can be found here -
http://dont-starve-game.wikia.com/wiki/Guides/Beginner's_Guide_for_staying_alive
Willow (firestarter)
160 XP
8 days
Wolfgang (Strongman)
320 XP
16 days
Wendy (Bereaved)
640 XP
32 days
WX-78 (Automaton)
960 XP
48 days
Wickerbottom (Librarian)
1280 XP
64 days
Woodie (Lumberjack)
1600 XP
80 days
Credit to ImJustYourDaddy :
By cooking and eating a mandrake, you can skip 2 days. So just make it so that those are set to 'more' with everything else other than what you need to make fires set to 'none'. I got 51 days in 20 minutes doing this