Well, besides all of the plentiful advice given so far, one thing that helps groups with spared levels is altering encounters a bit differently.
One idea that struck me when I read the first post was, "What if a group consisted of a fresh young 1st level with a lot of potential, and the rest of the group had varying degrees of experience?"
And this made me think back to The Hobbit and Lodoss War. In both of these stories, the main character was definitely not the strongest character in the group, but he was definitely important and served an equal part.
One way to 'help' the weaker characters is to leave opportunities for them to 'struggle' where the higher level characters would easily destroy an opponent. Depending on the actual role-play and thoughts of the characters and players, some encounters may seem absolutely trivial to the group. Here's my example situation:
Simple set-up, Goblins in mass attack a human village. They have vast numbers, many rogues and fighters in the mix, and other nasty trouble makers, but none higher than level 4. The highest level PC's are 8th level, and the lowest is a 2nd level fighter. As the group confronts the goblins in many different and varied battles, they work their way through multiple ranks and situations of battle, until only the shaman/chieftan/etc. is left. The 8th Level mage, who could easily destroy this challenge is, for some personal reason (or specifically to help aid the young fighter), refuses to fight it himself. Instead, he 'somewhat underhandedly' asks for the fighters sword for a second, then casts a stilled, silented, magic weapon or similar spell on the blade before returning it to the still healthy warrior and telling him to 'deal with the goblin'.
The fighter, hopefully, edges out the fight and unknowingly is aided by the mage, and we the sneaky DMs give the fighter the full 1200 experience just to him, working towards his catching up.
Of course, none of this is really directly applicable advice, but it's the thoughts that occured to me concerning the situation, inspired by recently re-watching the old Lodoss war boxed set.