I do that as a standard with all my management contracts. I'm not sure that's the unique proposition you are looking for.
Focus In on the Communication more. I manage primarily smaller properties (duplexes to 8-plexes), so I make a point to stop at every property every week. I then send a weekly update email to all clients of all issues impacting their properties, maintenance issues outstanding, new city legislation, etc. all clients also receive a monthly newsletter with their monthly report outlining the positive reasons for investing in our area, as well as a pocket listing that may lead to increases business for them as well as I.
Edit: One more thing. Focus on being one of the strongest members of your investor clients team. Have resources at your disposal to refer them to only the best accountants, bankers, contractors, etc. Make sure everyone on your team is as strong as you are. Make everyone shine. Your clients grow and so will you. I've said it before to others in the business locally. We don't move forward unless everyone moves forward.
Hopefully this all makes sense. Typing from my cell.