+1 on disassembling and coating the interior surfaces of the frame with a good rust preventative. If you really want to go that far, it is possible to rinse the tubes with mineral spirits and swab out the areas you can reach with some prep solvent or equivalent, let dry and then fog the interior of the tubes with rust converting primer using a long plastic straw. Eastwood makes a variety of products for dealing with existing rust and preventing future rust. Read the descriptions carefully as there are rust converters that must be used on rusted surfaces or they will not adhere, and there are rust converting primers which work as a primer on bare metal but convert any bits of rust stuck in corners or pores/pits into an inert material that will not corrode any further. Make sure your protect the exterior surfaces of the bike with careful masking prior to fogging the inside of the tubes.
Painted surfaces can be protected with a good polymer or polymer/wax blend automotive product. Meguiar's makes some good ones. Chrome should be treated with a good chrome polish/protectant, aluminum with an aluminum product that removes oxidation and protects the surface, and exposed steel with an appropriate lubricant/rust inhibitor.
It's not a bad idea to replace as much hardware as possible with aluminum alloy or stainless steel versions. All of my bottle cage and rack bolts are SS as are all my cables. Carbon fiber is another possibility for some components.
When installing BBs and other parts which are prone to sticking due to corrosion/oxidation/bi-metal reactions, a silicone based grease like Superlube or Park Synthetic is more resistant to water and salt than some other products. It is more expensive, but a $3 tube or $15 tub will last years.
Your greatest weapon is vigilance. Clean and protect/relube frequently and don't let corrosion take hold.
When upgrading your ride, you might want to look at a stainless steel or carbon fiber frame with as many anodized aluminum alloy, carbon fiber, and/or stainless steel components and fittings as possible. You still need to be vigilant as salt air is tough on everything.