That is what I do also. I add an extra thin layer of interfacing to the lining and also quilt my bag. It seems to give them more body and durability.

Same here. I also like fusible fleece.

I've used fusible fleece also.
As for the bottom, I use plastic canvas, one or two layers, and make a sleeve to insert them in.

Great tips!!

For the bottom of my bags I use plastic corrugated board - like the stuff signs are made of. I buy 8'x4' sheets at the Home Depot and cut several boards for storing bolts of fabric, and use the narrow leftovers for bag bottoms. I also cover them with a fabric sleeve.

I was going to suggest fusible fleece, then read all the other replies, with some great recommendations. Sewing with Nancy had a series on making all types of bags and totes, and she recommended using cotton duck, which is fairly inexpensive. I also bought plastic canvas to make some totes, as the ones I had made previously didn't stand up the way I wanted them to, plus I wanted to use something in the bottom of the bag that didn't have to be removed when the bag was washed. You got some great ideas here. Next time I've got a problem, I'm coming here for help.

I use fusible fleece on the outside of the bag and a pellon product named SF101 on the lining. It is also fusible and gives the lining the body it needs without making it heavy.

I made a Halloween Treat bag that I wanted to hold it's shape. I used fusible pellon and also sewable Heat and Bond. It was very stiff as you can see by the photo I've attached. I also sewed a separate piece for the bottom and inserted a piece of plastic placemat cut to fit. It turned out very stiff and held it's shape very well.

I used a pattern once that called for headliner foam. It is 1/8 " maybe a little less, foam with either one or both sides covered with something that looks a little like nylon tricot. It gave great body to the bag, it stood up fine and wasn't as heavy as the bag I used warm and natural cotton batting for. That is the only thing I use now, at least for bags that are supposed to stand up. I made one that was a slouch type bag with the interfacing recommended by the pattern, but only because it was supposed to collapse. You can get the headliner foam at Joann's Fabric (I usually us a coupon). I have washed my bags on gentle and dried on low in the dryer with no problems.

Someone just told me that used political lawn signs, made of corrugated plastic, make good bottoms for totes. I use Timtex (Peltex) or template plastic cut to size, but I'm going to see if I can't get one of those lawn signs from a friend who just ran for office. She lost, unfortunately!