It's easier to get a good finish if you keep your basic shapes simple. The further you go from the basic tube with a bottom concept the more variables there are to challenge your precision. Keep curves to a minimum and don't make them the centre piece. My favourite basic shape at the moment is a tube with a squarish oval base. I've varied the height and proportions of the base over time, but for ease of construction as well as overall polish this is my tried and true. It forms the basis for my shoulder bag pattern and fabric kit over at Ink and Spindle as well as several of my own handbags in recent years. I simple squared off base on a flap top messenger bag is also a good one, especially for kids.
Leather is the most durable material for making bags and despite what you may assume, it is quite feasible to sew leather on a regular garden variety sewing machine. But I can't say sewing leather is much fun. You can't use pins and you can't unpick which does create a certain pressure. For this reason I prefer using fabrics, but tend to sturdier ones - canvass, linen, hemp. I've also worked out that I am a serial monogamist when it comes to handbags and I tend to start thinking about a new one on pretty much an annual basis, less because the old has worn out than because I just get bored so longevity isn't my absolute criteria.
If you want a bag that looks like something more than a sack of fabric, whether its a stand up or a slouchy version, then I can't say it enough - interfacing, interfacing, interfacing. And not that cardboardy fused junk from spotlight. The first time I used a really good quality fabric interfacing was a revelation, it gives substance, strength, structure without taking away the fluidity and fabric characteristics of the outer. And there's no need to stop at one either, combining the individual characteristics of different interfacings allows you to add the exact combination of stiffness, strength, softness, fluidity etc that you want. My latest bag has no less than four different kinds of interfacing and structure - an all over treatment with a medium weight iron on fabric, some stiffening with Vilene to make the bag stand up well, some wadding to add softness and protect my ipad and some template plastic in the base to make it super sturdy on the bottom.
When it comes to lined bags there seems to me to be two basic construction methods - making a single piece out of lining and outer that you then turn right side out through a hole in the lining, and the make an outer and inner as separate pieces and join them through top stitching. The former is a must if your opening has a curve (like this), but the latter allows you to use much firmer structure without needing to scrunch it all up to turn it out through a small hole. It's also easier to my way of thinking.
You may have noticed I'm a real big fan of pockets and internal organisation. I love that a quick glance can tell me if I have all my critical things - an empty pocket alerts me to something that's missing.
I've played with this aspect of bag making and while each new bag seems to have more and more complicated add ons, I haven't yet gotten tired of a super organised system. This is absolutely a personal thing so if you want to go the pocket-o-rama route you really need to work out what stuff you always carry, how big everything is and your preferred balance between security and ease of access.
I am a fan of the webbing adjustable strap for my everyday handbags. I wear my bags messenger style most of the time but sometimes switch to shoulder bag style. Webbing can't be beat for ease of use and strength, and you can buy all the slides and loops in numerous styles for whatever width you want. Too easy. For softer bags (like the beach sling bag) I use an all in one outer and strap with a bit of interfacing for strength. I also like magnetic fasteners if I want a bag closure because they are easy to fit, strong and and can be opened and closed with one hand (though try not to have them too close to electronic equipment like phones and ipods/ipads since they can fritz the system).
I think that's it for my collected wisdom, such as it is. Do shoot questions through but I will answer them here in the post if I think they might be useful to others. If you want a personal answer just send me an email rather than leave a comment.