Coming from smalltown, USA, i've taken more sobriety tests than a judge.

You might think that what you need to do is keep your eye on the pen.
What the officer is watching for is the twitch of your eyes, or the movement of your head. He says "watch the pen tip" and he will move it from side to side and move it out of your range purposefully to watch you react.

Lean against your car. Cross your arms. Never admit to using a substance.

Once I took 3 sobriety tests from 3 different TEXAS TROOPERS. They couldn't pinch me for DUI/DWI as long as I passed the test. They let me go with a seatbelt ticket. As the trooper was filling out my ticket, he asked:
"what is your height?"
I said "6'10".
He said "don't you mean 5'10"?"
I said "yes, thats what I said."
He asked, for the 100th time, "have you been drinking?"
For the 100th time, I said "no"

If they want you to blow, never blow unless you are sure you're under the limit. If you have had a 12 pack, refuse to blow and suck it up and spend the day in jail. If you are high, just act disinterested. High people are "panicked" which is what they teach troopers/law.