The Mid-Game is when the fun really starts. You have a stable supply and income of cash, you might have scouted some with your infantry or whatever units you have (excepting Harvesters of course). You probably have a couple squads of infantry scattered around your base for defense. Now its time to plan.
By this time the Basic Build Order is done. Now you must decide if you are going to use the Offensive or Defensive Build Orders (or any other Build Order). Follow your planning, but remember that if you and your enemy are about the same skill-level, he might be taking the offensive route. A couple Combat Tanks/Quads could rip you apart if you are too busy scouting. A good rule of thumb is: by mid-game, be ready to kick ass. But mid-game is also when you have to start advancing through the tech-tree.
As for your level of military activity, have a squad or two of infantry at every entrence into your base, and put them on rock outcroppings if at all possible. You might want a couple Trikes/Quads with them in case you get a visit from other vehicles. If you can support these guards with turret-structures, more power to you.
When you begin to attack, don't attack and then suddenly stop. Press the attack. Hit important structures. Can you imagine how bad of a situation you could put your opponent into if you destroy his Refinery, or even worse, his Construction Yard? Take out a couple Wind Traps, or take out Barracks. Anything that poses a significant threat, take it down. And while your main force is attacking, make sure you are preparing another force. When the main force is demolished, bring in the next wave. He is likely not to be expecting it. And even if he is, their is likely not much he can do to stop you if your first wave accomplished its goal.
-Early-Game Strategies-Late-Game Strategies-Main Page-