ride it often and keep it clean and dry. if you're near the coast it can be a real battle. bikes on the porch of our beach house that weren't being used rusted baldy in a few years. years ago when other bikes were regularly used and cleaned rust was still a problem, but not nearly as severe.

inland is not as bad. I live 30 miles from the coast and the problem is greatly reduced. But it still exists, Especially under cables and around cable guides. and around bottom bracket lugs. rust begets rust so if a bike is left sitting in humid storage the rust can expand more quickly. if you ride frequently and keep it clean and dry you're less likely to have problems.

depends on the paint job as well. some bikes are more prone to developing rust due to the nature of the paint. and not necessarily cheap bikes. some nicer vintage rides are known to be rust magnets. if you're talking about new steel I think paint will generally be much better and rust should be much less of a concern.