I've often found postcards to be good for this kinda thing -here are a few I saw the other day or have stuck to my door:

Nice Person, Wrong Planet

I'm not wierd... I'm just special

Please leave this planet as you would wish to find it

(with a picture of a dog on the front) At least my dog looks at me when I speak to him...

Bankers are people who lend you an umbrella when the sun is shining and then want it back when it starts to rain.

And a personal favorite:
Nothing right in left brain. Nothing left in my right brain.