All great idea's posted here, especially packing the area's behind the wheel seals with grease, if there is no empty space then there is no where for something to get into.
The swing arm is hollow and even though there are plugs with O rings,water still gets in. So, you need to make a drain on the bottom of the swing arm in front of the chain block. Pull the plug on each side of the swing arm for the shock mounting bolt and, using a tywrap or other flexible device that is TOO long for you to drop inside, measure the distance from the access hole back to where the swing arm is solid. Come forward just enough to clear that solid area and drill a small hole on the bottom of the swing arm on each side to all the water some place to drain from and the air to get into to dry it. When returning home flush out this area to remove the salt.
Remove your rear axle chain tensioner bolts and coat them liberally with anti seize compound too.
Put a drop of a good penetrating oil like Kroil on each spoke nipple thread before and after you go.
As Steve mentioned about the grease earlier, I would pull the swing arm pivot axle and apply a good coat of anti seize compound to the axle and inner bearing sleeves as well as the inserts in the cases to keep your swing arm axle from becoming one with those parts. IMHO I think that anti seize is better than grease for this application. Same goes with the wheel axles too. But, would use only water proof grease behind the wheel seals. Try and make sure you don't leave any grease on the outside if possible as the sand will stick to it.
If you are going to be riding in the salt water a lot, then I would pull at least one vent tube from the carb up on to the frame some where so when you run through the water the vent tubes will not be trying to pull water in. You can also do what I did and cut in a T fitting to each vent (there are two vents, each vent has a T so that each vent has two hoses) and run a hose up to behind the number plate and then using a little 90 degree fitting make a loop so that the end of the tube is pointing down. Also, be sure and cover the vent tubes with a rag or piece of foam and tuck them under your skid plate to keep the finest grit from entering your carb bowl through the vents. This fine grit will make it's way into the float bowl and fuel and into your motor.
If you run through a bunch of water you may want to drain the bowl once or twice to see if you are getting any water in there.
Another thing I do to my bikes is to fill the electrical connectors with die electric grease. CRC makes it in a pressurized can with a small tube for a dispenser. Pull each connection apart and fill the female side with grease and push back together, then wipe off the excess.
Use a good chain coating agent like chain wax. It dries to a non stick film and wont attract sand. Use a grunge brush to clean off the heaviest debris at the end of a ride and coat the chain liberally with chain wax and you won't have a corrosion problem there. Eventually the wax builds up a bit on the chain, but is easily removed with a rag and some WD-40 or Kroil.
Of the two anti corrosion oils mentioned, Kroil will not evaporate like WD-40. And you can get Kroil in an aerosol, AeroKroil. The only problem with either of these used int he short term is that they will attract dirt, and will hold it there. I think Motul bike wash
http://volvospeed.com/Reviews/motul_motowash.html is your best bet as it cleans and leaves a light protective film. I have been using it for years with great results.
The only place where I disagree with the other posters is that I do not endorse pressure washers. Just the water pressure from a garden hose is sufficient for cleaning a bike. Pressure washers push dirt and grit into places that they shouldn't be. If you have some stubborn dirt hit it with moto wash and use a brush.
