* setup a personalized URL (PURL), and/or a QR code--displayed prominently on your billboard-- that will lead your customers to a highly-relevant landing page where they can read more about the offer and most importantly "take action." This online behavior should be tracked with analytics.

* give your customers the option to call: have a unique phone number on the billboard that is tracked (e.g. # of calls, duration of call, resulting sales) so that you have a clear view of your sales funnel.

* if you're displaying on a digital billboard, you should have the unique opportunity to do A/B testing to see which offer is pulling more action--lather, rinse, repeat.

* always have clear, strong and low-friction calls-to-action (CTA): make sure the consumer can answer the "why" in your advertisement (e.g. why should I call in, or "like" them on facebook, or inquire more about the offer?)

* depending on your organization/product, you may want to run an on-hold advertisement for your CS and sales lines, again the same concepts apply (PURL, CTA, measuring sales funnel).

* depending on the size of your brand and marketing budget, send periodic surveys and make sure to include a question about where your customers have seen your brand (e.g. billboard, direct, tv, radio, in-store, etc.)

These concepts will also work for direct mail, radio, tv, etc., except that with direct you can segment your list which gives you the opportunity to display/sell a more targeted offer and television/radio will be more of a broadcasted message, though there are niche time slots and programming options.

Another note: I don't really believe that you can run a truly, "offline" advertisement. When is the last time you saw a billboard without a URL? How many consumer-facing companies don't have some sort of (albeit poorly managed) social media presence. Consumers are so "wired" and digitally enabled these days, it makes no sense to run a siloed offline campaign because a) it's more difficult to measure--which I presume is why you asked this question in first place, and b) you'll get a poorer response rate. There are exceptions to probably everything that I've mentioned here, however, I use these best practices with my clients.

Can anybody add to my list or provide experience that contradicts my points?