Keeping in touch with your allies and trade partners is very important.
I'm allied with the French as Venice for about 150 turns. The relations were poor, though. I then offered some gifts, such as maps, single payments or small tributes. The relations improved to good in the end.
Every few turns or so I'll give my new map as a gift to the Pope. I'm always getting the "Of course, we accept this gift with open arms" and "You truly have our best interest at heart" lines.
Actively fighting the enemies of your allies should helps also.

R'as