Always ask / find out what the other person wants..

And put a lot of effort into finding out where you can add value.

Gives you a negotiating edge and lets you tailor your pitch accordingly. 

Seems quite obvious eh? As I learnt recently, it tends to slip our mind when we get too caught up in trying to get our idea across.. which we tend to do when we are really passionate about something.

So when you are really passionate about something and want another person to join you, make sure you actively slot in some "listening time". (Not so obvious now perhaps? :) )