I don't like styrafoam peanuts as they can make a mess if you have to pull a plant out of that pot, but you could put them in a mesh bag as you're just trying to fill up space in order not to use more potting soil than necessary. I also save the large styrafoam packing pieces that come with appliances, etc. Broken up into large chunks those work well. I sometimes also use some the lava rocks on the bottom. They're small, light weight chunks and a good size bag is reasonable to buy. I especially like these in my medium pots for cacti or plants that don't like the soil too damp as they provide good drainage. Plastic bottles, of course are great for huge pots. I put a piece of weed control cloth over all the drainage holes to prevent insects crawling in.
Like Jo Ann I remove a portion of the soil and replace with new potting mix. With my very, very large pots which have plants I hope I don't have to replace (like my Key Lime), I find I just need to add a little more mix every year.
