Say you are giving up alcohol for your birthday for charity. Every time someone offers to buy you a drink say you would like a donation instead. Have a pot behind the bar and make a thing out of it with all the customers. You could even keep a tally of the drinks people would have brought you and write them on a list, that way people can see how much you would have drank and also what it all would have cost. Put in some money you would have spent on drinks for yourself too :-)
At the end of the night you should have a decent amount and can then pass it onto a good cause. No lies being told and anyone that thinks you getting pissed is more important than giving it to charity can sod off :-)