IMO, the best way really is practice. After a few games then you can delve into strategy guides which will help alot. Also, try to play logically, not emotionally. Don't hold grudges, ex, attacking a player about to win is more important that continuing an attack on a stabber or something. While this may seem pretty cold, view players as numbers and opportunities, not as people. I was very trusting of people when I started playing diplomacy (still am...) and I almost always lost..
A thing that isn't so great about diplomacy is that sure, it involves strategy and negotiation skills, but it ultimately comes down to "who can lie better".
Anyways, its really fun, and the best way to get better is to just simply play.