I find that most people have no problems getting the very basics of RTS, but they just need a few key points to rev up their game to a much higher level.
First is that you can never have too much resources. In a game like Red Alert 3, that means taking expansion mineral nodes and derricks whenever you can do so without it being in total danger getting blown to bits. This gives you more resources per minute, which in turn allows you to get more units per minute.
Which brings us to the next pro tip-always be building units. Even when you are fighting and controlling , go back to your factories and what not and ensure that you are always spending that money. build multiple factories or barracks if you can't spend the money fast enough and rally them to a staging point so that if your attacking force gets beat back or you need to retreat, you can immediately come back and press the attack with fresh units.
This concept is known in RTS-speak as "macro", as in macroeconomics.
The second concept is to get out on the right foot in a game. This means following something known as build orders, which are optimal opening production timings for units and buildings such that you get the most out of the first few minutes of the game and put you in a good position to really have a strong army and/or economy going into the midgame. Offical game forums and places like gamereplays have forums where people share the best build orders for their sides and for the specific game matchup (say soviets v. allies or what not).
Finally, there is the concept of control, and this really comes down to nothing short of practice. This is the ability to know at an almost instinctual level what the next action you need to do is and as a result your ability to perform different actions at the same time across the game increases dramatically. Usually this is measured by "actions per minute" in the RTS scene and as you get better at the game and more familiar with both the interface and what the correct response to a certain strategy the opponent is using, you will be able to react faster as that instinctual manner of thinking takes over. This isn't necessary for single player or even casual multiplayer but in the RTS games where I've gotten into the top part of some ladders it's mostly been because I played the everliving crap out of them for weeks on end.
