If you're not predisposed to call people unless you have a specific question to ask or task to further, use your planner as a tickler file to remind you to reach out to the key contacts for your project.
Take the time to prepare for the call. Make a list of project-related topics that many of your key contacts would find interesting or useful. Add to this list, whenever possible, with tidbits that you get from the people you've called.
Make mental or physical notes to remind you of specific benefits and/or concerns that key contacts have. Whenever possible, reference them with some new information.
Always include an "Is there anything I can do to help you with your project work?"-type of question when speaking with participants.
If the key contact has a topic s/he'd like to discuss, be a very good listener. Avoid the urge to give a quick, pat answer unless you get a clear indication that quick answers are what they're looking for.
Don't prolong the call. If they're busy, be short, supportive and responsive (as in "Be sure to let me know if there's anything I can do.") If they're not busy, don't let the call slip into chit-chat if you can avoid it.