Anything that creates a protective film to prevent oxidation. Pledge furniture polish is cheap and effective. Oil based products like WD40 will dull the engine aluminum over time, but it's good for other areas like handlebars and exposed steel/chrome.

My favorite product is PRO HONDA SPRAY CLEANER & POLISH.

• Cleans and polishes all plastics, chrome, aluminum, and painted surfaces without water.
• Leaves a clean, streak-free finish with one application.
• Contains anti-static ingredients to repel dust and dirt.
• Protects finished surfaces from harmful UV rays.
• Leaves a long lasting high gloss finish.

Regardless of what you use, it's important to start with a clean bike.