Different bandpass filtering maybe?
Use some frequency analysis of the raw vocal and then set your EQ to place them in different ranges.

This is something I have banged on about a number of times on these forums, and I'm really glad you've started a thread that gives the topic more exposure.

The sound of a mic track is made up of a lot of factors: mic placement (height, distance from source and axis orientation as mics sound different on-axis vs. off-axis), studio acoustic and, crucially, the pre-amp used. An SM57 put through an M-Audio Fast Track will sound very different from the same physical mic put through a Neuman V496 or an API3124.

If you don't have the variety of different-flavour pre-amps to try, a simple experiment for vocals might be to use the SM57 (with a pop shield) and record the various takes with the mic set an increasing distance away from the vocalist's mouth. We're talking inches here, not room mic effects, so you could, for example, construct a series of thin wire frames to offset the pop shield in a forwards direction. Not only will the "proximity effect" of bass emphasis change over a distance of a few inches, the vocal quality will be subtly different as well. Bring the levels back up with a few extra dB of gain and you'll be surprised what a difference it makes to the multi-tracking of vocals without you having to alter any EQ settings. This works for almost any cardioid-pattern mic, but with less effect on something like an EV RE20.

Great ideas!

I approach this from a much different position. I'll utilize the same preamp. I'll utilize the same microphone. But I move the source to different distances with each subsequent overdub. This sets up a different timing & different acoustic characters and thus, no build up. The 57 is the most marvelous microphone to utilize as it really isn't much different from its more expensive sibling the SM 7. So what you are really hearing is a lack of different timing differentials. And so you get build up. You could move the source or you could move the microphone or both. The microphone will respond differently with different placements in the room and the microphone will pick you up with different timing differentials from different reflections within the room if you move the source.

It really works.
Mx. Remy Ann David

Thanks!

I'm going to have to experiment to hear this. Record the same thing the same way over and over.

I've never heard this buildup and I use only a few mics. Maybe I'm inadertantly doing the little fixes.