1) Interfacing & Interlining De-mystifiedhttp://u-handbag.typepad.com/uhandblog/2007/01/interfacinginte.html
Lisa explained which interfacing goes well with which fabric. She also shared how to avoid bulk at the seam (the same as si kurus suggested) and why use a lighter fusible interfacing than the bag's fabric. (It is so that any creases made due to normal bag use will not show up in the bag! this does not happen to sewn in interfacing though..)
Another very good link about interfacings in bag making:
2) Choosing Fabrics & Interfacingshttp://u-handbag.typepad.com/uhandblog/2009/01/bag-making-for-newbies-part-2---choosing-fabrics-interfacings.html
She provided suggestion of fabric and interfacing combo, here is an example, checked out her post for more combo suggestions:
Bag Type = Firm structure boxy handbag
Fabric = Quilt weight cotton
Interfacings = Medium Fusible on all bag parts and some stiff Ultra Firm Fusible on the exterior and Heavy Sew-in in between the exterior and the lining fabric, also there is a stiff grid bag bottom on the base.
There are many good posts on bag making for beginners:
1) If you wonder the meaning of the terms such as Right/wrong side out, Seam allowance etc used in the bag making instructions, see this:
2) How to strengthen - reinforce - or beef up your handbags and purses
3) Machine Needle know-how
Many more good stuff, check out her blog...
Thanks to Lisa for sharing her experiences and knowledge...