The following assumes you have resources to spare.

When you're at the point in the game where you're ready to attack, do the following FIRST:

1) Build 16 marines, 2 dropships, and 4 wraiths

OPTIONAL) Develop STIM PACKS

2) When the marines and dropships are built, hotkey two groups of eight marines and load 'em in the dropships

3) Move your army into attacking position, just outside of your enemy's vision. If you're unsure of the range, play it safe and only go about halfway between your base and your enemy's.

4) Take the four wraiths and find a weak spot in the enemy's defense. Have the four wraiths attack and slip your dropships through while the enemies attack your wraiths.

5) Drop the marines among the enemy workers

OPTIONAL) Hotkey both marine groups, and use STIM PACKS

6) Order the marines to Hold position

7) Attack with your main group

This is a dirty, dirty trick. Since the enemy will be forced with two choices:

a) Letting you slaughter their workers and thus cut off their resource flow

or

b) Diverting some of their defense force to killing your marines, and thus, dividing their forces.

CreepColony's Opinion: Make sure you get the dropships and stuff ready before you are ready to attack.