What I see in this game is penalties to trading given for Guarded and Hostile relationships (ie to get 1 extra lux resource from them, you need to give 4 lux resources, 2 strat resource, a chunk of gold) but ZERO bonuses given for DoF trading. This is a little bit unbalanced and it makes for a less interesting, unrewarding system of diplomacy. Sure it's nice to have some DoF and a good block form when you are going for a Culture victory, but that doesn't assure you of anything since Civ5 leaders can and will backstab if they see potential gain from it.
In future DLC or patches (or perhaps a mod), there needs to be rewards for maintaining a DoF. As previously stated by another member, the AI can come asking you for gifts and give you penalties or rewards based on your response, but you have no way of asking them for gifts. It is difficult to program the AI to understand what the repercussions
might be for either response in that situation, so perhaps just giving a boost to trades is the way to go.
In other words, if my civ is in a state of unhappiness (1-9

) and I have a DoF with Civ X, then Civ X's leader should be willing to trade one of their excess luxury resources for half of the value that it generally would call for (ie buy the resource for 250

becomes 125

since you are friends).
Perhaps this isn't a good solution, but there should be SOME form of bonus to trading when having a DoF.