Replace any and all plastic bags used to carry any forms of shopping with reusable cloth bags (and bring them with you where ever you go).
You probably get the idea, but a few notes...
I find it really important to love at least one of your cloth bags (since you can have several.) So don't buy just any bag, but find out which is aesthetically pleasing and you enjoy carrying around with you. This is ironic since you don't spend too much time worrying about what your plastic bags look like but that's just it; this isn't a one-time (or few-time) use plastic bag, but a durable bag (like a unisex purse)!
I have a few bags and I usually carry them around in my purse. I found this a bit challenging at first because I'd always forget to take them with me but then I made a small effort to make a habit of it. Guys can put these in a backpack or a bag they have or even just carry it around as a bag with your wallet and keys in it. I'm also open to other suggestions.
Now, even if I'm not going out shopping, I don't leave the house without one. If you have a car, this is even easier because you can leave a few of them in the car.
This is my favourite cloth bag:
It's from Marimekko (a Finnish textile and design company) and it was a gift from my friend Emilia.
The reasons I love this bag so much:
1. The design.
2. The colours (or shades if you're being specific.)
3. The fact that it's made from Silver-medal materials. (This means that the textiles were not gold-quality but close and were thus used for another purpose than their main purpose.) There are probably more eco-friendly bags out there but since I already own this bag and love it, there's no reason to get rid of it and replace it with another. While we're at it, Marimekko is also synonymous with high quality, so it's bound to last me many trips to the market!
Update: There's currently a lawsuit being launched by multiple plastic bag companies against Chicobags, a company which sells reusable bags, due to the content of their website. Their website has a 'Learn the Facts' section which explains why using reusable bags is more sustainable than using plastic bags. (Apparently this is bad for the plastic bag industry.) The petition can be found here: http://www.thepetitionsite.com/takeaction/191/658/839/