We made up a budget and stuck to it, we also did a lot of things ourselves, such as own invitations, used cake as dessert so only paid for one course at reception, paid cash for photographer and band. Also we bought our rings on special so got a really good saving.
A friend of ours were married in her parents garden and invited about 25 people and the week later had a massive party for everyone else and they said it cost around 4000.
Oh and make Ebay your friend! LOL! you can get shoes, dress, bombienerre (sp) etc much cheaper on Ebay
Good luck and remember its your day so do it how you and your DP want!