I've been experimenting with hidden plastic belts. I thought it might be useful to others to hear about my experience.
Hidden or "invisible" belts are made of clear, thin plastic and have a thin, flat buckle that's designed not to create a bump under your clothes. I decided to try one after seeing them mentioned here about a year ago.
Let me start by saying that I REALLY need a belt. I think it's because I have a straight, rectangular frame and basically no butt. I'm such a rectangle that I can take off my jeans without unbuttoning/unzipping them. Having a belt means that I can keep the tops of my jeans above my hip bones and my jeans won't fall off me. I don't wear jeans without a belt, because if I try the jeans slide down and would literally fall off if I didn't keep yanking them back up.
I've tried two different brands of hidden belt, called the isabelt (http://www.isabelt.com/) and the invisibelt (http://www.invisibelt.com/). Both belts work in a similar way. They don't have a regular belt buckle, but instead have a plastic clasp that joins the two ends together. This means that you can't really adjust the size while the belt is on - you have to do that ahead of time, by sliding a loop of the belt through a slider, like you do with an adjustable bra strap or backpack strap.
The isabelt is narrower, about 1/4" wide, and is the more stretchy of the two. I tried it first because I thought it would be less obtrusive. This didn't work well for me - in order for the belt to be tight enough to hold up my pants, it had to be so stretched that it rode up between the belt loops and touched my skin. I found that annoying. Also - perhaps because it is so thin and small - I quickly lost it.
The invisibelt is about 1/2" wide and is not very stretchy, more like a leather belt than a rubber band. This can be a disadvantage because the belt doesn't have a lot of give. If I change size, I have to adjust it. This works well for me because I can have it tight enough to keep my pants up and it doesn't ride up; it stays put better than the isabelt. The wider plastic also feels like it holds everything in place better.
Overall I think the belt works really well. At this point I've had the invisibelt for at least 6 months and I wear it every time I wear jeans. It keeps my pants on, and is much less bulky than a regular belt.
Although the belt is much thinner and less obtrusive than a regular belt, it can still be visible - if I wear a top that is form-fitting or has thin fabric around the waist you can definitely tell that something is there. For tops that aren't snug it is usually impossible to notice the belt.
Another advantage of these plastic belts is that you don't have to remove them to go through airport security - they don't contain metal. So now I can keep my pants on while I travel.