I've found that there are two types of Diplomacy players when it comes to this sort of thing: there are those that "don't make plans" and those that "make plans".
To identify these groups a little more, I'll lay down some rudimentary guidelines. Those that make plans are the types that immediately come out and spell out ideas. They propose a way forward, they lead and they direct. Those that don't make plans are the ones that sort of "go with the flow"--whatever that flow is. They will always make plans for themselves, but typically don't initiate plans with others. They aren't shy in counter-offers or negotiations, but they simply aren't the ones to break the ice.
Let's assume some scenarios for a moment: Let's take any two individuals that fit into one of the two categories and examine the results.
A) Both don't make plans--When two individuals begin talking and neither of them are plan-makers, topics often revolve around cordialness and they often wait for the board to develop and examine the results. They'll make DMZ requests, talk about things they don't like, but will generally not form a cohesive plan and will primarily do their own thing. Alliances are formed, but opponents are often slower to break down as one person merely requests support thinking of his or her own plan and the other will either agree to give it or state that their unit is busy doing other things. These alliances tend to last quite a bit longer, and even possibly game-long, although there is very little communication between the two.
B) Both make plans--This is sort of like two ranking officers deciding the way forward. Either one will have a stronger say in the matter, and one will have less say in the matter, or there will be quite a bit of disagreements when two people think that their way is better and the one that should be followed. Plans often work, but neither party is completely satisfied and one person will typically be more on edge and there will frequently be tension among the group as each member thinks that the plans aren't going quite as smoothly as they had expected. Alliances are formed, but can quickly deteriorate at the drop of a hat.
C) One makes plans and one doesn't make plans--This type of dynamic is the "preferred" dynamic. One player will lead while the other will follow. Opponents are dissected with the fluency of two players that lead, but both parties are satisfied with the results. These types of dynamics, however, can often lead to the brutal stab of the player that doesn't make plans if they do not carefully examine the situation to check and make sure they're not getting hosed.
I tend to be of the "makes plans" type. I noticed that people will often prefer to work with someone that makes plans over someone that does not make plans. I first start by looking at what I want in any given situation. After I have discovered that, I try to think about what the other player will want and try to offer that thing. This often ends up in the scenarios described in B and C.
Ultimately, I think the goal should be to develop a "B" scenario where one makes plans and the other doesn't necessarily make them but might counter the plans with another set of ideas, but someone among the group needs to be coming up with something. If neither of you are, then it's simply important to change something up by (a) knowing what you want, (b) knowing what the other person wants, and doing what you can to ensure that you both can get what you're after.