Keep in contact with everyone
Good communication with everyone is essential in Diplomacy. Talk to everyone, even to your enemies or the players that are far away on the other site of the map.
When your are behind, diplomacy is even more important because an avoided battle is a battle won.
Keep also in mind that your today enemies can be your tomorrow friends.
Stay under the radar
When you are intimidating much other players, when you play flamboyant, when you open the game very successfull or when you make outrageous claims, other players will make you a target.
Be humble and helpfull or the underdog. It's not important to be the biggest power from the start. You must be the biggest on the end.
Tell the truth as much as possible
There's no point in lying needlessly. Players who lie much get quick a reputation for being untrustworthy.
The key in Diplomacy is telling the truth without telling people what they need to know.
Have at least one friend and an enemy
Without friends, it's impossible to survive.
Having a public enemy is also important. You need at least a target to attack and this way other players feel more safe next to you as they assume you will focus on your enemy.
The enemy of my enemy is my friend
Remember that even players on the other site of the map (or enemy neighbour) can be valuable to you.
Also think that the friends of your enemy are potential enemies, even when they are your friends.
Try to avoid the hold order
A unit that moves can do the work of two. Self standoffs, cutting support, the scissors and the schizophrenic support are a wonderful innovative options of possible orders.
Offer help to players who are weak
Try to help another player to survive and much chance he will reward you with help later.
Helping others to finish in third or even second spot doesn't hurt you.
Stick by your allies
While you are walking toward victory, make sure that your ally gains some supply centers too.
Betray an ally only when it's really needed. Think carefully about it and don't do it early in the game. Betrayed allies often go for a revenge.
If you betrayed an ally, then write at least him a mail to explain why you did it. If you are very convincing, he might forgive you.
Look for allies on the far side of your enemy
The best allies are often those who are on the opposite side of your enemy because you and your ally usually will not get in each other's way.
Second, neighter you or your ally will snatch unprotected supply centers from eachother's backfields.
Play for fun and shake hands after the game
Diplomacy can be addicted. The community is large and most of them have long memories.
After intensive games, don't carry your grudges. Don't take things personal. Take also care of your own reputation as this will reward you in the long term.