I almost always play two characters. Usually they will be related somehow, and I do trade them out on occasion for story spotlight purposes.
Example? Brother and half-sister team of Jarl and Amberlee MacAllister. He's a Human Fighter/Rogue/Shadow Warrior (homebrew PrC; kind of a mix between Master Thrower, Ninja of the Crescent Moon, and Duelist) and she's an Elven (formerly Half-Elven -- long story) Favored Soul/Soldier of Light of Taelisha (PrC borrowed from the back of Deities and Demigods; Taelisha is our homebrew Goddess of Light and Healing [and my oldest D&D character deified *woo*]).
When it's time to kick down the door, kill a lot of stuff, and be generally loud and obnoxious, I play the 16-yr old Jarl to the hilt. When it's time for quiet diplomacy, common sense and a touch of morality, 21 yr-old Amberlee takes centre stage. In dungeons, I take them both and they work as a team with their "pet" NPC's/supporting cast.