Keep Your Troops:
Keep those troops alive. You'll never survive the later missions if you haven't nurtured your people and grown them into Exarchs.
Only Fools Rush In:
Knowledge is power. Only fools rush in, so use your Mimes to scout the opposition. This will keep your army from stumbling into nasty ambushes.
The Right Equipment:
Give your best people the best equipment. Just as you always plop that 10-3 Captain on the heavy machine gun, you must also stuff the best artifacts into your Warlock Master's backpack.
The Right Tool:
Use the right tool for the job. Pay attention to each unit's special abilities. Units with armor-piercing weapons should be used against enemy armor, while those with Scatter Lasers work best against the bad guy's infantry.