Hi,
One of the first steps to become a master (Pbem) with this kind of games is to actually know when it is the right moment to "retreat".
The "art of retreat" in the hands of a skilful tactician, is nothing less than a tactical redeployment for a specific reason:
1. Redrawing or adjusting the defensive perimeter
2. Redeployment due to overwhelming odds (breaking/gravity point of assault)
3. Redeployment in order to launch a counter-attack.
Basically the last point is the ultimate scope of every retreat...
"Retreat in order to prepare a counter-strike" 
.
Thoughts, anyone?
cheers,
Pyros