Controlling your units is important. The ability to control your units well means that you may, with a smaller group of units, overcome a larger group of units controlled by a less skilled player. Some of the control tips that I am about to mention you will most likely know. Others which are less well known, you may not.
Harrier/Black Eagle Scouting - Scouting with harriers? Yes! They are fast and have a good range of sight. The problem that you most frequently experience with them is telling them where to go all the time isn't it? Because with these units you can't use waypoints. But you can with harriers. This is how you do it; you click on waypoint mode and then set the waypoints on your radar screen. If you try doing it normally it wont work , this allows you to tell your harrier to scout all over the map extremely quickly. This allows you can to get on with what you want and leave your plane to reveal the map.
Harrier/Black Eagle Kamikaze - A useful trick this one. To make your Black Eagle/Harrier hover over a building you just use ALT and LEFT CLICK. It will then hover over the building and the enemy air defenses (assuming they have them) will shoot the plane down and make it land on top of the building. Planes cause considerable damage when they land on top of buildings. Try it out and see for yourself. Although it's a costly trick it can be deadly, allowing you to wipe out opponents buildings in one swift attack.
Infantry Squasher - This allows you to wipe out infantry very, very quickly. To squash infantry with your tank press "alt + x" and then click on the desired unit to run over. Your tank will squash it quickly with minimum of fuss. This is much quicker than trying to squash a unit manually or simply trying to shoot at them. Remember that Tesla Troopers cannot be squashed. This tactic is especially useful for Rhino tanks when they are up against groups of GI's.
Scatter Units - Press "X" to scatter a group of units (they must be selected). This is useful if you want to scatter your units when they are being fired upon by V3's or a tank is about to squash them etc. Also try using it just before a harrier/black eagle fires their missile at you, this manoeuvre will often cause the missile to miss your unit.
Advanced Waypoint Usage - Try selecting your units then selecting units/structures as waypoints. They will then go and destroy them in the order you selected. A good control tip as it decreases the amount of time spent on micro management.
Grouping - A very important feature of the game. If you select various units to be put in a group then you can call upon that group at any time by simply pressing a single button. This is particularly important if you have groups of harriers/black eagles sitting in your base waiting to attack. By grouping them you can press one button then the control cursor will appear. This saves you a lot of time as you don't have to scroll back to your base then mess about trying to select them when you could easily select other units in the base by mistake as well. A vital skill.
Target Selection - When fighting your enemy, for example, in a huge tank battle, try not to use all your tanks to kill one of your opponents tanks, each tank only takes needs 4 hits to kill it. There's no point in clicking 30 of your tanks on 1 of your enemy's tank and hence wasting 26 hits which could have killed around 6 of his tanks. So what you do is select around 3-4 of your tanks to kill 1 of his tanks and repeat this across the battle line. This way by the time he has killed 1 of your tanks you will have killed around 6 of his.
Distraction - When you are fighting your enemy tank to tank, a very useful trick is to use dogs as a distraction. What you do is build around 4-8 dogs depending on how big the tank battle will be, and put them at the front of your tanks. Just before you send your tanks in, send in the dogs. Quickly follow with your tanks. This will make the enemy tanks fire at the dogs while your tanks move in for the kill on his tanks. This also works even if you send in the dogs half way through a battle. You will find that tanks often have a tendency to fire at dogs.
Summary: Competent control is vital. Good players know how to group their units and how to control them. Try practicing grouping in co-ordination with some of the control tips that I have just mentioned.