I'm not an expert on this, but you want to make it a reasonable request (money, product or a combo of both), and remember the costs involved in putting their name on your car somewhere (decals, clothing, etc). You want to help them promote their product/service, and by doing that you want to tell/list why you're a good candidate for that! Keep in mind that a business wants to advertise with whatever brings in customers, and while some are geared more towards certain forms of advertising, others may not want to get into racing only for promotions. Car shows, promotions for the business, and getting their name(s) out are others forms to use. Most experts agree you should have a presentation prepared (on paper and speech/pitch form) with what level(s) of sponsership you offer, and what costs to the business are at each level (levels can be determines by amount of work you do for them, size/placement of decals, etc). Check out National Dragster, as they have a column from a motorsports marketing manager every month that gives tips on this subject.
Good luck!
Derek