In reply to Ndawson:
Used to do this all the time when I lived in Scotland. My strategy for success was to consider areas with easy public transport (Glencoe, the Northern Cairngorms, and Fort William were high on my list), or as Jamie says to make friends with people who own cars!
Wild camping or the use of bothies also gives you more flexibility, but of course you need multiple days to get anything done if you're using that approach. I've been known to bivvy at the side of the road or in car parks when necessary.