The principle is to total up an amount equivalent to the value of every drink drunk regardless of whether you paid for them or had them bought for you. So rather than cutting out those (vital!) morning caffeine oral injections and risking the potential of a skullcrushing headache this little spreadsheet sums up all your drinking at home and/or out and about during Lent. The final sum can then be given to a suitable good cause, my preferences are Microfinance sites such as Kiva or, my personal preference, UK based Deki. The template for this year (2012) is available to download here which even allows those with a Catholic disposition the option to leave out Sundays ;-)
An interesting side effect of this process is that this does enable some self discipline both in moderating drinking habits and reducing mean-spirited attitudes!
Meanwhile for another approach during Lent check out Tearfund's Carbon Fast: