There are many levels of a stiffer interfacing you can use. Experiment until you find the one that works best for you (meaning provides the strength but is still easy to work with). One thing is to remember to not include the thicker ones in your seam allowance. Sometimes it is easier to sew that seam using your zipper foot. I find that when making the bags that look more like a man's grooming bag (sort of a small mailbox look), I like to only reinforce the bottom and the ends only allowing for the curved sides to be a bit more 'soft'. And, it is a good idea to do a couple lines of stitching thru all layers (even the fusible) to make sure they are secure and don't later move around. For the zippered pouches (like make-up bags) I do not use interfacing. I use either clear vinyl sandwiched between cottons or I use the 'babyville' fabrics for the lining (they are coated as to be waterproof). I find these give plenty of structure and provides a leakproof barrier.