Keeping the party at 1-level gap at most is probably a good policy. There are several ways to help do this:
1. Figure XP as if all the PC's were of the lower level, divide by total number of PC's, and award them to the lower-level PC's. Then go back and figure things normally for the higher-level PC's.
For instance, 3 level 8 PC's and 1 level 7 PC deal with 2 EL 9 encounters in the current session. 4 level 7 PC's would have earned 8,400 XP, so the single level 7 PC gets 2,100 XP. 4 level 8 PC's would have earned 7,200 XP, so the level 8 PC's get 1,800 XP. The level 7 PC closed the gap by 300 XP.
2. Go back to the "classic" method, taking away 1 Con point each time a character is raised from the dead rather than losing a level. If you feel this doesn't "hurt" enough, take away 2 Con points (this ALWAYS hurts). By the time the Con gets unacceptably low for the player, he will probably be ready to create another character anyway.
3. New PC's start at the beginning of the highest current party level, and any existing PC with less than that gets enough XP to match the new character. This is kludgy, but sometimes death teaches the survivors something in a way that might earn XP.