Snagged from a post on the Swords & Wizardry boards by Telecanter: http://www.swordsandwizardry.com/forum/viewtopic.php?f=10&t=1175
Save progression for all classes is identical; start at 15 and advance as core rules. Classes get bonus to saves depending on effect:
Clerics +2 versus divine or unholy effects
Fighters +2 versus incapacitating effects (sleep, paralysis, petrification, etc.)
Magic-users +2 versus magical effects
Thieves +2 versus dodgeable risks (traps, breath weapons)
Simple and extensible.