The Council for British Archaeology is a charity who aim to educate about archaeology and help make it more accessible. So helping them out means they can help more projects, raise more skilled hands willing to work for free or cheap and generally promote something not seen as valuable, since unearthing bits of pottery isn't as 'hot' as preserving stately homes.
http://www.britarch.ac.uk/supportus
or, if you would rather donate to something you think is more 'worthy'
http://www.macmillan.org.uk/Donate/Donate.aspx