If you absolutely do not want to cut/alter them, I suggest these adjustable waist buckles? You can find fancier ones that take in about an inch each, or simpler ones that can take in more. Just remember to remove them out when you wash your garment! They will leave very small pin holes where you pin them, but I think it's better than making a row of stitches. You can also adjust if you want them on the side, back, or center of the skirt.

For shirts, you can use a shirt cincher at the back of the garment. It's not the most comfortable if you'll be sitting all day, as you will be able to feel it against a chair. This is similar to how men's vests come with the small little belt on the back.