I've just sold my car (Audi A6 Limited Edition, high miles but top condition and a relatively high asking price compared to others)
I started off on Pistonheads, but after 2 weeks, lots of calls but no viewings I put it in Autotrader (£45), that improved interest and I got a viewing.
What really got things moving believe it or not, was putting it on eBay (£15) which I had previously ruled out due the low asking prices for most cars.
Got a calls the same day (a Sunday !) 2 days later I had a full price buyer and 2 standbys.
My son reckons it's because it is so easy to find cars on eBay's site whereas Autotrader is a pain and Pistonheads has limited coverage.
Also I always find that launching a new advertisment in the 3rd week of the month is more successful as it gets noticed by people who have just been, or are about to be, paid !
Finally touching on previous comments, if you can offer viewing in a covered and lit location, put this on your advert.
Good luck, hope you get a sale soon