Edited the OP to make it easier to read. Would be great to have some more faction-specific tips for factions besides OKW, if anyone has any good ones.
USF:
-You don't need to have your priest crewed, while on cooldown. You save up mp and popcap by just crewing when you need to barrage.
-If you are using AT mines from Infantry/AssEng put them on groups of 3/4. Debuffs don't apply with 5+ mines.
-Airborne MG and AT gun breifly gives vision. I'm would love seeing someone make use of it someday.
OH:
-You can deploy smaller patches of minefields by simple moving away after each batch of mines is built. This let's you customize how big you want it to be.
-On the same way, minefields are cheap and awesome for sinking tanks on ice.
-Target weakpoint for Stugs/Tiger Ace actually target tanks in comparison to paks.
-Vet3 + Spotting scopes 222 is a constant maphack.
SU:
-You can cancel your initial building to fake your tech early on. This can also be done midgame for T3/T4.
-Good idea on putting demos behind LOS, specially on buildings. People "never" turn the camera.
-Vet3 T70 using the scout ability is on the same level of vetted 222 + spotting scopes.