First reusable bag...your old bags. Cost is free, usable until they fall apart. I do forget my reusable bags from time to time, I am only human. When I do forget, I think about the stash of bags I have in the pantry and think about what ones I am running low on. I use the paper bags for my weekly recycling and a few for gift wrapping and craft projects. I use those multiplying plastic bags for poop patrol with the dogs. The kids use them to haul their winter gear to school in too. I have also started "collecting" a few more for the upcoming garage sale.
Second reusable bag...Chico Bags. These were the first bags I actually bought, and still use them today. I bought them at ReusableBags.com and paid $4.95 each for them. I got five, and I literally fit a full Cub Foods cart of food into them, minus the milk. I purchased them back on 4/4/07 and have used them soooo many times. I have used them for groceries, camping trips, dance competitions, caring food at holiday times, even caring toys for the toddler. My oldest son and I used to count how many bags we saved by using them, and completely lost track after 100. I would guess we are in the 200's by now. The best part of these bags is that they fit into a small carry pouch that is attached to it. I can fit all five bags into a medium sized purse and still have room for all my other stuff. Plus now they come in fun colors, I may need to get a few of those.
So these are the bags that I personally have. There are thousands of other bags out there. There are even some for free, like the free Earthbound Organic Bags. I have not received mine yet, but I am waiting patiently. Some stores do have free giveaway days, so be on the lookout for those. But the trick to making the bags work for the environment is to use them. Leave them in your can, carry a tote bag full of them, do whatever you have to do to remember to use them. Then think about how many bags you are saving!