You need to focus on seeding your main subject matter keyword in your ad and then find long tail keywords to supplement the ad as well. Google LOVES content that reflects the subject matter. ie:

Car
Blue Car
Audi
V8
Acceleration

These are all keywords that compose the main subject of car and thus will attract Google even more to the page. Use either a free keyword tool like google’s keyword search tool or a paid one, like keyword research professional.