If it's email, my experience has been keep the touches valuable and consumable. By that I mean, don't send them a 2 page email with all kinds of content about how awesome your company is and why they should use your product. Take the education marketing approach. Send them little nuggets of useful information or data. If you are selling an analytics platform, give them some insightful analytics they can immediately benefit from e.g. "did you know that 50% of people who walk on a car dealership lot end up buying a car in the next 30 days... be sure to capture contact information from EVERYONE that steps foot on your lot".

The recipient of that email drip feels they got a real valuable data point that is also actionable and they'll remember it came from you. When they're ready to buy, you'll be top of mind.

Links to articles are also great for drip campaigns e.g. "hey, we saw this interesting article on customer patterns for car dealerships and thought you might find it valuable" -- of course, if you guys did the analytics and produced the white paper that's good too.

Key is to mix it up. Frequency is ok, if it's useful.