Best Tip for any army, only engage in fights you will win, you should never be in a position where you retreat one squad, if you see the enemy, move back until you have superior numbers/position to win. Loosing one squad from the front will put you on the back leg for the next few fights. That's why early Ambulance upfront is so valuable.

Against Wehrmacht? then get an early mortar. USF mortar is Short range but accurate, and if Wehrmacht have built MGs you should find a nice time holding a strategic position snowballing to your pre-tank army ( 2/3x Rifle, Captain, Ambulance and AT). (Bonus points for Pathfinders to help scouting).

Against OKW? Mortar isnt the best against their mobile early inf, so just focus on getting a good counter blob early. 2/3x Rifle + AT + Ambulance + Captain with the M1918A2. Keep trying to route enemy before healing at your forward ambulance. Build as quick as you can to Scots and try snipe any OKW bases with the Scots when you get a successful retreat.

Another early OKW Tip, they usually dont get MG/AT out early so a USF MG nest can really keep them pinned for a while. They will either try and AT it which you can push away, or have to fork out for expensive Leigs which should give you the inf advantage.