Like other people said, you pretty much just have to try on lots of things. If you can, take someone who knows what looks good to check. Most people do have at least a couple of odd measurements. It’s even more of a pain in the ass if you’re obese, athletic, or really skinny; most clothes are made for “average” people, who are moderately overweight and don’t have much muscle.
Jeans come in lots of styles. I have the opposite problem from you. I can’t wear regular straight leg Levi 501s or similar because my thighs and butt are too big. A nominal 30 waist usually fits properly, with the right amount of slack, but I can barely squeeze into the legs. If I go to a 32, the waist is huge and the legs are almost comfortable. The last pair of jeans I got that fit just right—not too tight, but not loose—were 505s. They fit me like 501s do on most other people.
Try different fits, different manufacturers. I can’t wear Dickies at all because none of the proportions in anything approaching a proper waist size fits me, in any of the styles they make. Jeans themselves vary even when the nominal size is the same, so try on every pair of them to make sure they actually fit.
Shirts are easier if you’re looking for dress shirts. You can actually measure your neck, shoulder yoke, torso length, chest, and arm length to get a proper fit. Most dress shirts state neck and arm length, some have a more detailed fit. The standard large, medium, and small sizes don’t work as well because they vary a lot more.
My problem with shirt fit is that I’ve got long arms and I’m in decent shape. Something that fits in the arms and chest is usually baggy around the waist. If it fits just right in the torso, the arms are usually too short, too tight, or both. I’ve had some success with some brands of “fitted” or “tailored” fit shirts. You’ll have to try several kinds on to see what works for you.
If you’re anything like me, you hate shopping because it’s hard to find things that fit without trying on what feels like half the crap in the store. Even having a few trusted brands doesn’t save you from having to try things on to confirm, but it does help to narrow things down a bit.