i'm not catholic, but i enjoy the secular challenge of giving up something for lent. in 2007, i decided to give up shopping bags (i.e., any disposable bag given to me when i purchase something). three years later, i've never gone back. and it's actually pretty easy if you follow these four rules...
1. tell cashiers right away
this is the hardest part of living bag-free. cashiers are so well trained to bag your items that they'll slip it in before your good intentions have a chance to react. the only way to prevent this is to get yourself in the habit of saying "i don't need a bag" the second you put your items on the checkout counter. if you forget and they happen to bag your stuff, don't be shy about asking them to take it out. don't worry. the bag isn't sullied. they can use it for the next customer. trust me.
2. always carry your own bag
this part is much easier for women because they usually carry purses, but lots of manufacturers make reusable bags that wad up to the size of a film canister -- easy for pocket portability. i usually carry two reusable bags in my purse for everyday use, and i have a few more in the trunk of my car in case i do a big grocery run. i love my greenaid (photo below) because it zips up into a tiny ball, but i think they're kind of expensive (i got mine as a gift). i also recommend the fold-up bags from modcloth (photo below) -- they fold up really small, but they're very sturdy with comfortable handles.
3. be dexterous
look down. see those five-fingered things sticking out of your arms? they're called hands. use them! i hate to see people get a bag for a pack of gum or a soda or leftovers at a restaurant. if you can put it in your pocket/purse or carry it in your hands, do that. you don't need the extra trash.
4. reuse your mistakes
if you accidentally get a bag, it's not the end of the world. just make sure you don't throw it out right away. i use my roommate's paper bags to collect recyclables in our apartment, so we can just dump the whole bag into the recycling bin every week. plastic grocery bags can replace trash can liners if you buy one of those grocery bag trash cans (available lots of places, but here's one from the container store).