I haven't made up my mind yet - might give up table salt (I bring my own shaker to every theatre I work at), or soda (I'm a Cherry coke and Sunkist gal).
Another Lentian strategy I'm considering is a combo: to give up / give back. So it works like this. I resolve to give up eating out - but I know realistically that I will get take-out probably once or twice a week anyway. But every time I spend money on unnecessary or indulgent food (including the random slice of pizza, Chinese with the crew) I must give the same amount of money to a charity. So my $7 sandwich = $7 for my charity. I keep a running tab in my desk, and at the end of Lent, write a check for the total. I've done this in the past, and it works for me. This way, I'm giving something up for Lent but also giving something back to the world a little too.