If you're on GDI and your Barracks is down, a Grenadier rush can be very effective, as
shooting a building on the MCT with the grenade launcher does as much damage as if a tank were shooting at it.
3 of them can easily take down a building, and it won't cost you a dime.
Actually organize rushes! I have found myself in games recently where I will go "lets do a flame rush guys"
and the next sequence of vehicles I see is a light tank, an arty, an apache, a stank, etc.
Everything but what I called for.
Communication is key. A simple "APC incoming!" can be the difference between losing your refinery and holding off the attack.
There is no shame in being an engineer/tech/hotwire and repairing vehicles/structures. It's a critical part of the game,
and without someone doing it you will lose, as every vehicle needs support.
Take the field! Or at least try to. If you don't have the field the odds of you winning are not very good.
Constantly being on defense repairing buildings is not the way to go.
Attack the harvester immediately. Get a few engies and a few soldiers and just C4 the shit out of it.
You want to prevent the enemy team from getting that critical first credit drop off, which means while you're buying tanks they have to sit and wait, which is a huge advantage.
Hillcamping, a legitimate strategy, even though a lot of servers even had rules against it.
But, a team of arties or MRLS's on the hill in Whiteout (Hourglass) can decimate the base,
or at least keep everyone on the enemy team occupied with repairs.
Humvee/Buggy rushes. You and a buddy in a buggy/humvee early in the game can take out a building if you do it properly, and it is a devastating blow to lose a structure at the beginning of a game.
A group of Mammoth Tanks is useless. Enough said. Have about one or two mammies backing up some meds and MRLSes.
Mine your buildings. One of my favorite things to do is be a ninja and slip into bases unnoticed,
and do my trademark "Waltz in unopposed and blow up a building with ease" and it is a fan favorite
of many other players. 4 simple mines in a doorway will prevent this from happening.
If you're playing on an AOW server, points are very, very important.
In my opinion, in an early rush, you should target the refinery or power plant.
Even on a map with no defenses, the doubled production cost early on
will definitely hamper the enemy team's ability to quickly retaliate.