^
Also, tip cleaners, or acidic fluxes (anything with ammonium chloride, zinc chloride or similar stuff in it). Obviously enough, the salts or acids form ionic (conductive) residues. They're also much more strongly acidic than rosin (which is, after all, an organic acid, much milder and less volatile than, say, vinegar), which will corrode the joint much more quickly, especially with humidity around (zinc chloride itself is deliquescent, meaning it absorbs moisture from the air so strongly as to form a liquid solution of itself).
Still occasionally worth keeping in mind, for those strange occasions when you have a hunk of dull copper around, or you need to solder to steel. Tin it first, using the nasty stuff, then clean it up as well as you can!
Often, good old mechanical cleaning does as well. I've used coat hangers for electro-mechanical purposes (namely... the stuff makes good self-supporting antennas, stiffer and cheaper than heavy copper wire). Sand or file off any coatings, rust, etc., and try to tin it as soon as possible. Exposure to air causes oxidation, and it's a time * temp thing (steel will start turning yellow, then other colors, as the oxide layer grows). In this way, I've had no problems soldering bare steel. (You're welcome to try the same procedure with stainless, or aluminum, but have fun with that. Most recommended methods for those require the ridiculously nasty ammonium bifluoride based fluxes. So, better off with nuts and bolts for those materials.)
Tim