I am thinking about buying a LeMond Zurich with an 853 steel frame. The only thing I am concerned about is that I have heard that steel frames and salt and ocean air don't really mix. I live right on the coast, so this is an issue for me. So, my question is, is this really a problem and if so, can I prevent it?

I don't think it's such of a problem that it needed to be even considered.
If bike is not going to be laying on the beach, unwashed, for months, there will not be even possible to notice any signs of "ocean related corrosion".
I have a mid 70-s Schwinn, that most likely spent most of its life here, in LA, and the bike is rust free.

I wouldn't worry about such problem.

I am from the coast(Oriental NC) and you gotta keep it clean. You need to check the frame often and touch up any chips and I would reccomend using Frame Saver. You spray this stuff inside all the tubes to protect them from rusting inside out.

My bike lived for 2 years outside in a UK seaside town, 1 mile from the sea. Every night I would ride along the seafront, whatever the weather. During storms I would catch a lot of sea spray. The steel bike is still going strong.
You need to rustproof the inside of the frame, with framesaver wax (or wd40), and protect the outside with a car wax. I also use wax to protect cables and the exposed section of bottom bracket, which can pick up a lot of rust.

We have four (4) steel framed bikes here on the coast and have no problems with rust. As suggested above, Frame Saver the interior of the tubes every few years and wax the exterior every season and you should be fine. If you think you've gotten salt spray on the bike then rinse with fresh water and dry before putting away. Touch-up the paint periodically. Rust is not a problem given some simple and easy maintenance. Same that I would do regradless of where I lived. Have fun!!

The Alien face is right; I use to live in Santa Barbara California and one of my bikes-the beater commuter-was a low grade steel bike that sat outside for 10 years in the rain because I lived in an apartment and had my good racing bike inside and there was only room for 1. I also rode that commuter bike on the beach where the tide was at and it would get drenched with salt ocean spray. I would take it home and rinse it with fresh water but I never treated it with any internal stuff, only waxed the outside about 4 times a year. I still have this bike, though in total disrepair. There is some minor surface rust on the rear and front dropouts and where the paint is scratched on the frame. but nothing even close to serious. This bike is now 22 years old.

My other steel bike is my main bike and it to is now 19 years old and it has no rust on it anywhere and it was never treated with any internal treatments either.

As Paramount said, keep it waxed and touch up scratches. If your going to sleep over the rust issue than treat the inside of the frame with Frame Saver or BioShield; but the frame will outlive you.

WD40 applied to the outside of the frame is not a good idea, since it attracts grime and dirt like a magnet than you have to wash the crud off. Just wax it with a good (Mothers or McQuires) auto wax.

I live 1 km east/downwind of the Pacific Ocean and ride steel frames exclusively. Chrome plating deteriorates rapidly, and light surface rust is a frequent cosmetic problem, but otherwise everything holds up pretty well.