Resizing a PSD file?

I need to resize a .psd image from 15MB down to min of 10MB, what would the best way be to do this, I have already resized the image to the required pixel dimensions, so cannot reduce anymore there, what next can I do do to reduce the file size but still retain the integrity of the image as much as possible?
Thanks in anticipation...Lilly

Turn off compatibility mode if it is on. (no need for extra thumbnails, etc...)
Flatten the image if you don't need those layers.
Save in another lossless format - or save as JPEG.

Hi Lilly,
I am not sure, but i think the strenght of psd files is the abbility of saving in layers. And therefore you lose the abbility to resize picture files as in jpeg's or compressed tiff's. Compressed resizing as in jpegs always means degrading PQ after each save. And as PSD is the native photoshop file, i reckon the boys from Adobe won't want that to happen. Maybe a Zip tool like WinZip could do it? But i don,t know what that does to the IQ.
sorry,
Martin.

If you just need the layers, try saving in TIFF - it might be smaller
HTH
Rob.

And as martin said, compressing to RAW or ZIP would also decrease image size, maybe 1mb or 2.