If you play skirmish i know a way to have a great defense... set the tech level low enough to just about get pillboxes (no tanks ect) and build lots of these around your base and areas were you harvest and any entrance to get near your bases constantly maintain this and make gaps for basically gates to let your troops in and out.
This should keep you safe and allow you to make constant attacks or build up a massive army to wipe the enemy out....
Add more tips in here!!!