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
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.