Many people do their weekly groceries at large supermarkets and consume a lot of plastic bags. However, given the fact that I already know that I am going to buy a-week worth of food and supplies what can I do to avoid plastic bags?
Some countries have introduced a tax on plastic shopping bags. For example, in Ireland the tax is 0.15 EURO (about 12p in English money or $0.20 in American money). Within a short space of time, shops offered customers a choice between: (1) free paper bags, or (2) heavy-duty reusable bags (usually made of cloth, but sometimes made from thick plastic) for 1 or 2 EURO per bag. The traditional thin, disposable plastic bag virtually disappeared from shops. I suspect that the financial incentive of "I don't want to have to pay a few EURO for bags to take my shopping home" caused lots of people to remember to take reusable bags with them whenever they went shopping. And I also suspect that that is the goal the government had been hoping to achieve (rather than to raise lots of money from the new tax).
So, one way to solve the problem on a large scale is for the government of a country to introduce a tax on plastic shopping bags. If your country does not already have such a scheme in place to give you a motivation to remember to take reusable bags with you when you go shopping, then you could introduce something akin to a "swear box" in your household. Whenever you return home with a newly acquired plastic bag, you pay a fine (say, 1 EURO/dollar/pound) per bag into the swear box, and periodically you donate the contents of the swear box to a charity.
Reusable bags, mesh bags and a personal shopping cart!
One can use the mesh bags for fresh produce and loose items that needs to be scanned and the reusable bags to group the rest. A shopping cart comes handy because it is much easier to roll the reusable bags. It also provides a second option if any of the items don't fit the reusable bags, one can always toss them in his/her own shopping cart instead of any plastic bags.
Having a personal cart also saves time from finding/ returning the store's cart back properly if not leaving it buy the side of the parking lot and contributing to a messy clutter of shopping carts.
Tip: A knitted mesh bag with large holes works great for loose items like apples and oranges that needs to be scanned at the cashier...
I think you are referring to forgetting to bring the reusable bags when you go shopping.
I have the same problem and I think the easiest solution is to always have one or several bags on you. For me this means that my reusable bag always stays in my bag pack, which I take out everyday anyway. Depending on your situation, that might mean to put them into your handbag or into your car, depending on what you always take with you.
It may seem stupid to carry around an item which you might not use, but a bag or two is really lightweight and if folded correctly takes up almost no space at all.
For me this made really the difference of not using any plastic/paper bags any more. In the event that I do forget the bag, I "punish" myself by carrying all the items awkwardly on my hands, which usually makes me remember to take it next time.