Bicycle Mechanics - Rain preparedness?

I started biking again in the past couple weeks, after a pretty bad Northeastern winter. It rained here in Jersey yesterday, while my bike was locked up outside. What should I put on the chain to prevent rust? I was thinking applying some WD-40 before I go out could help, and I have a ton of that lying around. Are there better things to put on a chain/bike to keep it nice (eg, lube) and what would those be, and how much should it cost? Thanks in advance.
Edit: It's a steel frame

A bike cover will protect the whole machine. Just lube the chain as usual, but more frequently.

WD-40 is better than nothing, but not really good as a chain lube. But chalin lube is a fairly contested subject, do a search and you'll find loads of threads with various recommendations.
As for the bike as a whole, there are dedicated products that can be used to rustproof the inside of the frame, or you can simply pour in some linseed oil and slosh it around. For the outside, a regular car wax will do a good job on the painted surfaces. A modest amount of pretty much any oil (wipe off excess) will do wonders to preserve any exposed metals.

Don't use WD-40 as a lube. It'll remove rust-causing water (WD=water displacement), yes, but doesn't leave much lubricant behind, which can make the chain more vulnerable to rusting.

Check my signature below...

Use a frame saver, twice a year or whatever the directions call for +1.

Let it dry, skip the ride the day after.

Clean the bike constantly or use a car wax ...either way, turn the bike upside down each time.

...but if you have to leave it outside, you didn't mention what the situation is ...get a cover.

Please forgive me as I indulge in a bit of shameless self promotion. I try not to abuse the privilege but it seems apropos here.

Read these 2 rain/wet riding blogs and draw your own conclusions.

Big Bikes blog, June, 2009 (http://wellonabigbikeya.blogspot.com/2009/06/chain-l-chain-lube-half-assed-product.html)

Big Bikes followup, Sept. 2009 (http://wellonabigbikeya.blogspot.com/2009/09/chain-l-belated-follow-up-review-i.html)

There are others besides, but my point is that pre-treating your chain with a decent chain lube means you don't have to sweat rain.

I'm in New York & ride my commuter bike 100 miles per week, rain or shine, and do absolutely nothing except fix flats for 3 months at a clip at which time I still don't do much except re-oil the chain. (I use old Sun Tour "Grease Guard" hubs, BB, headset and pedals, so all they get is a squirt of grease when the notion strikes)

you didn't mention what the situation is
The situation is that I'm in school, and the only place to store bikes is the bike rack outside. How big would a cover be, and could I fit it into a jacket pocket/backpack?

Frame saver inside the tubes requires a BB tear down
most practical to strip the whole bike down .
since you have to rotate it around a lot to slosh the coating around.

cover? , a flat tarp turned into an envelope should work .

regular cheap blue ones , tape the edges up to close it front and rear..

meh - jus do a little gear cleaning post ride and keep it lubed

A bike cover would be nice but sometimes it's more of a pain dealing with it for packing and also it's prone to coming loose and blowing away.

I never had any chain problems in the wet with just using a heavy oil but then I didn't store my bike outside either. It had covered and heated parking at both ends of the trip. If a thick'ish oil for the chain isn't enough to keep rust at bay due to being outside all day then consider using a motorcycle chain lube. These go on thin and then the carrier solvent dries and it leaves a tough greasy lube behind. It causes a slight bit more drag in the chain if you flex it in your hands but I doubt if you'd notice it when riding. And it forms a very water resistant film that will last a long time. Unfourtunetly it's also sticky so it holds grit. So overall I found that in really bad conditions I wanted to clean and relube about once a week to avoid a gritty sounding chain. That worked out not badly since by that much time of steady rain the rear brake sounded like it was sandpaper on the rim anyway. Hopefully you have a place where you can soap down the bike and rinse it as well as a place to do the chain cleaning and relubing without it staining the living room carpet or any such sillyness.