Some years back I found something at Walmart that was marketed as a trunk organizer. It's basically nylon, foldable with semi-rigid sides and two handles, with two interior sort of dividers that can be velcroed to the sides to serve as dividers or left undone. It fits three to four standard-size reusable bags easily, so I can carry three in at once. And surprisingly strong. I looked around and I couldn't find an exact replacement, but something like this: http://www.amazon.com/Classic-Accessories-Veranda-Jumbo-Tote/dp/B005FTM8Y2/ref=sr_1_65?ie=UTF8&qid=1363027958&sr=8-65&keywords=bag+carrier
-- looks about the same and strong enough to handle a fair bit of groceries if you pack the bags in there carefully.
What I do is once my bags have been emptied of groceries, I give them a shake and a brushoff to make sure they're not dirty or accumulated cat hair (three cats.. yeah) and put them inside the carrier, folded up, and put the carrier on the hope chest I have sitting under my coat rack. Then when I'm ready to go to the grocery store, I just grab my coat, bag, keys, and the carrier - and out the door I go. I take the entire thing with me (it's lightweight) - I can put things inside the carrier on top of the bags, then unload at the register, fill up the bags, put bags back in the carrier, and go.
As for estimating how much you'll need - you can do a "dry run" at home. Pack a bag full of whatever things you normally get and test for weight and bulkiness. Add on a couple bags above that for things you don't buy regularly but might at some point. An insulated bag isn't necessary for me, since I live pretty close to the store, but useful for those of you who are further out or have particularly temperature-sensitive items. When my precious carrier finally gives up the ghost (five years and counting!) I'll probably invest in a wheeled cart.
And no, there's no harm (other than cost, and Walmart routinely sells their standard ones for fifty to 75 cents) in having extra bags. They can be useful for other things than groceries! Picnics and things like that. To help you remember to use them, do what I do - keep them where your keys are, so you'll grab and go. I also, for a time, had a post-it taped to the inside of my car window. "Do you have your bags?" So I'd remember to reach back and grab them. Then once it became habit I took down the post-it.