I would blog often and consistently. Some helpful topics would be:
1. Current OEM Promotions
2. Current Dealership Promotions
3. Service (tips, maintenance schedules, promotions, etc.)
4. Dealership Events
5. Dealership Life/Happenings (people love an inside perspective into your everyday dealings)
6. Your happy customers! Include pictures!
7. Ratings and reviews about your OEM lineup.
The blogs I follow most are interesting, insightful, and consistently updated. Those ones stick out and have me returning each day.
Also, I see a lot of dealer sites using Wordpress as their blogging platform and I think it's excellent. One bit of advice, it's technical, is to put the blog on a subdomain (blog.yourdealersite.com) and just point the subdomain to whoever is hosting the blog. I see far too many dealers purchase standalone URL's for blogs, but it can be done on your main domain too. I assume the reason is that some dealer website vendors don't have blogging capabilities built in so the only method they thought was to buy a new domain. If you need help with that technical part you can PM me and I'd be happy to help.
