Don't forget to mix it up!
If you have a website devoted to car parts, and it's been making a killing for years, also make one for totally unrelated stuff like Easter Bonnets
You never know if your breadwinner product will dry up.
If you get good google rankings with your car parts site, optimize other sites for other search engines. If google suddenly drops you for car parts, you're still #1 for easter bonnets at Yahoo.
And you won't be stung as hard.