I keep mine in the car trunk. But I still forget it half the time.
I do that too.
At smaller stores like Trader Joe's I've actually told the cashier to keep ringing up my stuff while I dash outside to get my bags.
At first I kept on forgetting to put the bags in the car. So now, I have a system: When I put away the groceries, I systematically put the emptied bags in one of the bags. When done. I hang the bag of bags on the front door knob. Now I can't forget to take them out to the car next time I go outside.
Also, I used to keep individual store's reusable bags separate, which, of course, got to be too much of a hassle. So now, I'll take Walmart and TJ Maxx bags to Whole Foods and Whole Foods bags to Trader Joe's and won't think anything of it. On the whole, though, Walmart bags are too small and flimsy, and TJ Maxx bags are too big for groceries. Whole Foods bags are made better, and I really like TJ Maxx Homegoods bags. What I really want to do, though, is to go back to the cotton canvas bags.