Advanced tactics

I made this thread so we may collect and discuss some more "advanced" tactics for new players to read up on once they've covered the basics. 

What I got in mind are some tactics and tricks not too obvious at first glance which can make the difference between victory and defeat when facing a similarly skilled opponent

Soo, to get things started:

If things drag on, get creative:

• Go after his economy and force your opponent to multi-task. 
• Do invest in upgrades. They can make one hell of a difference if chosen well
• Get the resource amplifiers in later game (don't make them in early game since they take a while to make and you need the resources elsewhere)
• Your armies you used for capping won't just magically disappear. Feel free to use them as decoys <insert evil mastermind laugh here>
• Radar has a very short range, especially on moving units. Use that to your advantage to sneak behind enemy lines. 
• Don't be scared to build factories somewhat close to the frontline. 

Toss some of your own tactics in as well