I can't personally vouch for any of these, but: 

Sign on an electrician's truck:
Let us remove your shorts.

Sign outside a radiator repair shop:
Best Place in town to take a leak.

Maternity Clothes Shop:
We are open on labor day.

No Smoking area:
If we see you smoking we will assume you are
on fire and take appropriate action.

On Maternity Room Door:
"Push, Push, Push."

On a Front Door:
Everyone on the premises is a vegetarian except the dog.

Optometrist's Office:
If you don't see what you're looking for,
you've come to the right place.

Scientist's Door:
Gone Fission. 

Taxidermist Window:
We really know our stuff.

Podiatrist's Window:
Time wounds all heels.

Butcher's window:
Let me meat your needs.

Used Car Lot:
Second Hand cars in first crash condition.

Sign on Fence:
"Salesmen welcome. Dog food is expensive."

Car Dealership:
The best way to get back on your feet -
miss a car payment.

Muffler Shop:
No appointment necessary. We'll hear you coming.

Hotel:
"Help!" We need inn-experienced people. 

Dry Cleaners:
Drop your pants here.

Sign in an office:We shoot every 3rd salesman,
and the 2nd one just left.

Veterinarians Waiting Room:
Be back in 5 minutes. Sit! Stay!

Music Teacher's Door:
"Out Chopin." 

At the Electric Company:
"We would be delighted if you send in your bill.
However, if you don't, you will be."

Beauty Shop:
Dye now!

Garbage Truck:
We've got what it takes to take what you've got.

Computer Store:
"Out for a quick byte." 

Restaurant Window:
Don't stand there and be hungry,
come in and get fed up.

Bowling Alley:
Please be quiet. We need to hear a pin drop.

Cafeteria:
Shoes are required to eat in the cafeteria.

Music Library:
Bach in a minuet.

Funeral Home:
Drive carefully, we'll wait.

But I have seen the side of the septic tank company's
truck, with it's slogan "Your crap is our bread and butter"