I need to add more belts to my wardrobe as well - but as a fellow hourglass (on the plump side), I've found that just about *all* belts work for me. It seems I need different belts for different bottoms though. Here are some examples of what has worked nicely for me so far in order of preference/priority:
1.Medium width leather belts (not stretch) - especially for jeans and pants, but also will work over skirts, dresses, blouses *and* shirts.
2. Wider stretch belts - these are fabulous to wear over knit dresses - ditch the "self belt" that some of these come with and put a nice, wider stretch belt over the dress instead. Bonus is that they are very comfortable, flex with you when you move, and the best part yet (for me anyway) is that they help camouflage any flab that might be bothering me.
Meanwhile, I just discovered that I can also pair the wider stretch type of belt with a button down shirt worn over a pencil skirt (not tucked). This seems to raise the button-down shirt up a wee bit, closer to the right top-length for skirts, and I get a nice semi-peplum effect from the shirt being worn untucked with a skirt. Note: I wouldn't wear a stretch belt with pants or with something tucked in. Seems to me the belt moves away from the waistband in that case (eg. wearing stretch belt over pants...belt rides up and is no longer on the pants!)
3.Skinny belts - these look nice on top of a pencil skirt when I have a blouse/shirt tucked in. It's not really to hold the skirt in place, just adds a nice flourish. These types of belts also work well over my very dark blue wash, slighlty dressier- looking flared jeans - again, if I want to tuck in a top. Trouble is? The belts that go over these jeans needs to be a larger size than the the ones I use over pencil skirts. The pants sit lower on my waist than any pencil skirt would - so yes, I need varying sizes.
If I were just starting to try a few belts - I'd get a wider width stretch type of belt and some medium width leather non-stretch belts. While I like the look of the skinny belts, they don't get worn quite as often. They are more like a decoration, and don't seem to have any additional qualities besides just 'finishing off the look' of pants or skirts. The stretch belts actually *do* something if that makes sense - they make flab 'go away' a little bit, they make knit dresses look a bit better, and as I recently discovered - can also be worn over a button down shirt. Plus, they are very comfortable to wear, and move with you. Meanwhile, I would consider medium-width leather belts in both casual and dressier iterations an essential item. I don't really like seeing empty belt loops - I know it seems silly, but to me it seems unfinished somehow.
Just remember that not all of your belts that you wear with pants will work with skirts and dresses. Annoying, but true! I'm not sure if this is just an "hourglass" issue or if the same is true for all body shapes and sizes.
Final note: I just went and measured my favourite stretch belt and it's 3" wide, whereas one of my favourite medium sized casual leather belts for jeans is 1.5" wide. Hope that helps!
ETA: I have no idea if I am short or long-waisted. I have at least two other stretch belts that are more like 2" wide. Just so happens that my favourite is the 3" one.
