You shouldn't have to add soil to potted plants constantly and frequently. Some soil loss immediately after potting is normal. A very small amount lost gradually over time is also normal. Your situation might be normal for freshly potted plants, but you can probably reduce it in the future by improving your potting technique.
First, look around the base of the containers for signs of soil leaking out. Most pots will not let significant quantities of soil leak out the bottom, but you should check to rule this out. If a lot of soil is leaking out of the pot, you may want to loosely cover the drainage holes. You can remove the plant to do this, or just wait until next time you put a plant in this pot. (If you wait until next time, you'll just need to add soil to the pot on a regular basis.) There are lots of different materials that can work for this - old pieces of broken terracotta pots, flattish rocks, a layer of sphagnum moss, several layers of burlap, coconut (or similar) fiber matting, etc. It can take some trial and error to get the holes covered enough to keep soil inside, but still allow appropriate drainage.
Assuming you're not losing soil out of the bottom of the pot, your soil is just getting compacted. This is pretty common with a freshly potted plant, because the soil starts out fairly loose, with lots of air space. Just add more soil to the top, pat it down a bit, and water it. Add more soil as necessary to reach the appropriate level around the plant.
When you first pot up a plant, jiggle and tap the pot to help the soil settle. Keep adding soil until it's at the appropriate level around and over the plant roots. Water it thoroughly, and expect it to settle a bit more when you do. Add some more soil if necessary.
Once you get the right soil density, you shouldn't need to add soil very often. But over time the plant will use up some of the soil mass, and you will occasionally want to add a bit of extra dirt. This can be an opportunity to refresh the nutrient contents of the soil as well as replacing the missing bulk. Try adding a mixture of soil and compost (fully composted material only, nothing that's still decaying), either from your own compost pile, or a product you can buy from a garden center such as mushroom compost, composted manure, leaf humus, etc.
