You don't specify where "outdoors" is; is it in your back yard? out in front where just anyone has access? in a carport where there is at least a roof? These all are factors in an accurate, helpful answer. Let's go for the intermediate--your back yard without a roof: in this situation, a large tarp and strong rack would be a good investment; I would rig an arrangement that will hold the bike frame (so that the tires are not on the ground). Then cover the whole setup with the tarp; be sure to secure the tarp so that wind doesn't undo your care. If you do have a roof, rig a couple of bike hooks and hang it, cover and all. If it's where the public has access, then I wish you luck!

I rarely kept any of my bikes sheltered from the elements and they all have been fine for many years. How did I do it? I watched out for the chain, bearings (in both wheels, crank, pedals and headtube), cables, and derailleurs. These are the areas most likely to suffer corrosion from being exposed to the elements. These are also areas that require regular lubrication for best performance. Well, if you lubricate these areas regularly, the elements will never contact bare metal; it won't rust/corrode. This is especially true for the chain, which is the most rust-prone. I also occasionally wiped the spoke nuts with an oil-soaked rag to protect them without getting oil on the braking surfaces on the sides of my rims. (Not all bikes use steel there, but some do.) I have never had any problems with my rapid-fire shifters as they are relatively weatherproof. The rubber on my brakepads never had a chance to deteriorate, as I wore through them rather rapidly, though if you ride gentler than me, this might be an issue. One last thing. If you seriously scratch the paint on your frame or forks, do something to cover it up and protect it from the elements or else the frame will oxidize.

Are you going to be riding your bike in the winter? If so you may want to keep the brake levers covered (with a road bike anyway) as water gets into them and freezes, making braking impossible. This for some reason doesn't seem to happen with mountain bikes.

I would suggest covering it up with a car cover. This should help to prevent any rust.

There are rain covers you can buy that are just like car or boat covers. That is the best thing I know to do on outside bike storage.

You can buy a plastic raincover in most cycle stores.

Pick a spot where the bike will be out of the direct sun most of the time. Between the house and a tree would be good. If you can arrange a couple of bricks or tiles, even a plank of wood to keep the bottoms of the wheels off wet ground that would be good too. You can buy bike covers from cycle shops but any large piece of waterproof fabric would do as long as you can stop it blowing off. Allow some ventilation to prevent condensation. Go over the bike with an oily rag, not dripping oil but greasy as often as you can. Don't do that on brake surfaces though.