although rims are a wear item (they ARE the brake rotor as was mentioned) I have seen maybe 2 or 3 outright failures in 15 years of wrenching in bike shops. the one I remember was from a racer using lightweight rims. so don't fret too much about it, you'll never wear out a rim from braking.
what I always did on brake tune ups was use isopropyl or simple green on a rag and scrub the rim down good and if I wasn't installing new pads, I would clean them as well, and lightly sand them flat. look at your pads; you'll find them shiny w/ bits of aluminum imbedded in them. not good. make sure the brake pads are slightly toed in and contact the rim squarely and evenly.
don't discount the need to keep the cables well lubed as well. the housing wears out also, increasing friction where you DON'T want it- inside the cable.