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Buy as much RAM as you can stuff into the machine and hope for the best. If that doesn't work, start saving your pennies for a new computer.
 
RAM, RAM, RAM! Also upgrading your processor, gpu, adding a RAID and/or extra hard drives (and setting application scratch to a spare drive), buying a larger start-up drive and only keeping the OS on it (so it is 90+% free space), minimal install of OSX, running disc utility and basic maintenance scripts as often as possible... That's really about it, but all of that (or some of it) will help.
 
Few.. dare a say some searching on the forums will come up with all that info for you? I dare.
 
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