There is no one universally best relationship marketing mode. Whether on the "giving" or "getting" side, people have varied communications abilities, comfort, and preferences. The most effective strategy for setting up your relationship marketing campaign, therefore, is to use and mix several communications modes. You can also call this layering.
If you mix one-to-one meetings with networking events, phone calls, IMs, text messages, social media messages, e-mail, greeting cards, and perhaps working together for a local charitable cause, for example, that does a lot more for relationships then sticking to one mode. At the same time, of course, you have to figure in your available time, your location, and certainly your comfort level. For example, if you live several thousand miles away from someone, you won't be meeting them in person very often, if ever, and if someone with whom you are forming a relationship heartily dislikes IMs and text messages, you're better off skipping those modes for that person.
You don't want to barrage other people with constant or intrusive messages or contacts, and it's most effective when you use contact modes you enjoy yourself, but the point with this tip is to consider your marketing strategy and identify those modes or media that will work best for your and your business. Preferably you'll also have the ability to dial up or cut back on specific types of communications as you learn other people's preferences.