I've found that the best thing to do is fill your trenches with soldiers, mainly infantry and machine gunners and let the enemy come to you for a little while, then shell them, let them do a charge, and after that wave has been defeated send out assault troops and as soon as they reach a trench, send all soldiers in that trench foreward with them. Then the next trench then after that start sending regular infantry. Another tactic is whenever you're equally matched with the enemy and both sides trenches are full, or at least yours, send out snipers to harrass the enemy. But always keep your trenches full, it helps.
