1) One Front: In general, a power should limit itself to one offensive front in the early to mid game. A two front war usually leads to a power's early demise as its forces become stretched too thin. The dispersion of armies makes it easy for countries fighting on their home territory, close to reinforcements, to repel and even invade the attacking power. In addition, a vacated homeland is an easy invitation to a stab.
2) Garrison: It is advisable for a power to leave a small garrison to defend its home command centers. These units serve two purposes. Firstly, they prevent a sneak attack on a power's home command centers. Secondly, they discourage an ally from making a stab. An ally will usually stab if they feel that they will make easy gains. The best way to prevent a stab is to make it more costly to attack your command centers than an enemy's. However, there is a delicate balance to be kept. One must balance one's offensive and defensive needs. The garrison must be large enough to prevent a stab or sneak attack but small enough to allow a strong attack.
3) Alliances: One must always have at least one alliance in the early and mid game. No country can win on its own and no country can repel a combined attack on its own. In addition, the two theatres which the early game breaks into are made up of three countries each with Italy as a wild card. An alliance with one power, if genuine, can tip the balance of power in that theatre in your favor. An alliance is especially important if a stab is being planned. Most likely, the stab will not wipe out the victim and one will be left with a very angry power that wants nothing more than to destroy the power that stabbed them. An alliance with another power can reinforce the power of the stab, if properly timed, and help to defend against the victim.
4) Secrecy: In general, share as little information with the other powers as possible. In Diplomacy, information is worth as much as armies. As such, however, information can be worth a lot on the bargaining table, and it can be worth it to disclose plans if it can get you other benefits. Be careful, however, about what you disclose.