Patrol a few scouting units all across the map, using the Shift key and the overhead map to set multiple waypoints. Revealing mana sites is the immediate benefit. Scouting will also let you know how aggressive your opponents are. You'll know the extent of their lodestone building, what kinds of units they are producing, and when they are attacking you. Be sure to set the waypoints for your scouts in neutral or friendly territory, if you can.
Aerial scouts are most vulnerable when they are making a sharp turn.
Once you can see the surrounding area, send units that can build lodestones out to your frontiers. Build lodestones from the frontiers inward and fortify the outer lodestones as soon as possible. Assign scouting units to guard your lodestones to provide line of sight for your defensive units. As mana allows, send more defensive units to the frontiers. Keep the pressure on your opponents, and you'll not only rake in more mana, but you'll keep them flustered.