These aren't focused on promoting third parties, but here are a few more ideas:

"True propaganda goes unnoticed."

"America is a free-speech zone."

"Wake up and smell the endgame."

"Would you like your TYRANNY in RED or BLUE?"

"Vote against BOTH EVILS"

"Trusting your party destroys America."

"The lesser evil gets away with more evil."

"The two party system is the wasted vote."

"Declare your independence! Vote third party."

Vote against the two evils...vote third party

"Vote one or they are, they hate competition!"
"Annoy a sleeper, vote for Spoiler"

"The government should be afraid of its people, not the other way around".

Shorter the better

"Enough is Enough : 3rd Party 2008"
"No Hope No Change until 3rd Party is heard"
"Third Party 2008, Stick it to the man"
"Vote Third Party, show them you care"

"The government should be afraid of its people, not the other way around".

'Who is John Galt?'

but how about:

'Dr. Paul said this would happen'

'I have to choose between taxes and war?! Why?'

'If leaving Iraq is such a bad idea, how come my pants were made in Vietnam?'

'Shouldn't the 13th Amendment have stopped the 16th?'

'Your plan is to stop Socialism with a trade embargo? That's retarded'

'Try and tax my garden. Try.'

'Try and tax my solar power. Try.'

'Bring the troops home . . . . even from Switzerland'

'Let me explain to you the difference between currency and money'

'The Fed does WHAT?!!'

'The Rs and Ds are exactly what I expect from public education and high fructose corn syrup'

'The Presidency should be a part time job'

'Ludwig von Mises could kick Karl Marx's ass'

'Bastiat was right. Marx was wrong. Keynes was an ass.'

'When you subsidize poverty, you get more of it. Duh'

'But I don't want to buy bullets for Georgia'

'Photons are free, seeds are cheap'

'That 'L' in front of my name means I'm smarter than you and have better guns'

"I voted for Ron Paul, don't blame me"

"Would you like your big government socialism chocolate or vanilla?" Vote 3rd party '08

"The Founders warned us against political parties. Now we know why"

Vote with your feet: Kick the Bums Out! Vote 3rd party

"My child's government approved education is superior to your child's government approved education"

Okay, im going off track here - sorry couldnt help it.