How do I remove colors from gif palette?

[cdev...@my-deja.com]

I converted an image from RGB to index, and checked the option to
remove unused colors from the palette. I'd like to remove even more
colors. In Windows I used Adobe ImageReady (which is really Photochop
with web features added). With ImageReady, I can sort the remainding
colors by popularity (frequency of use) and remove unneccesary ones one
at a time until the image is small enough yet not so small the image
looks bad. I haven't seen a way to sort the remaining colors then
remove them using GIMP. If someone knows a way or they know of another
program, please let me know.

P.S. It should also be known that I am making an animated GIF and
used several layers (but that shouldn't matter).

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