Will flashing ROMs multiple times degrade the performance of phone? If yes, is it possible to retain the performance? If no, then is there any procedure or sequence to flash a ROM on a Samsung Galaxy S?
If you always wipe the device between ROMs (clear system, boot, cache, etc [not
data]), then there should never be any degradation of performance. While flash memory does have a theoretical write limit, you won't make any sort of noticeable dent in the life of the device by constantly switching ROMs to try out new ones.
However, if you don't do a full wipe, you could see performance issues creep up. More likely, you'll run into bugs and random crashes as files left over from a previous ROM conflict with the new ROM. This is why you should always do a full wipe if you are installing a brand new ROM.
As for how to flash a ROM for your particular device, xda-developer forums for the Galaxy S are a good place to look. They list instructions on how to root the phone, install ROMs, and other helpful guides.