We'll be visiting for 3 days in August and will possibly have 2, but definately at least 1 car for the duration, which we probably wont use but will just park up.
Can anyone recommend a hotel without exorbitant parking prices in the centre of town (probably stay at, or close to the hilton near union square), or a nearby, reasonably priced parking option??
Thanks!

If you don't mind leaving the cars near the airport and taking BART in, ParkSFO is around $11 a day for outdoor parking, and they regularly offer coupons for one free day on a 3 day stay. BART would be less than $5 from the airport.
http://www.parksfo.com/

The City's parking site is on line with a list of garages and rates:
http://www.sfmta.com/cms/pgar/garages.htm

if you don't mind parking outside of the city centre, there is plenty of free parking in the Potrero Hill, Richmond, and Sunset districts public roads.

If you do opt for non-metered street parking in one of the neighborhoods, be sure to check the signs for permit requirements (they may be in effect on one block but not the next) and street-cleaning days (parking is prohibited for a few hours on a certain day of the week; days and times vary widely). SF is a very ticket-happy city, so watch out!

Your best bet if you're determined to park in the city is probably an independent garage, as the hotels charge around $40/day (more for "oversize" vehicles such as SUVs) and the city-operated garages can also get pricey. There's a garage near Mason and O'Farrell streets near the Hilton Hotel that doesn't cost too much.

I like the SFO suggestion if you don't have much luggage.

A hint for finding street parking in San Francisco - go uphill. Generally the nearer the top of a hill you get the more likely you are to find parking.
My last 2 places in the city had no off-street parking yet I found parking every evening with very few problems (and this is in an area of the city reputed to have tight parking)