One clever sticker I saw recently made me chuckle. On government vehicles you'll see the lettering ใช้ในราชการเท่านั้น "For Official Use Only" below the logo for whatever department it belongs to. A taxi had adorned its bumper with: ใช้หนีราชการเท่านั้น "For Fleeing (from) Officials Only", a play on the usual phrase by replacing ใน /nai/ with หนี /nii/.
If you're sensibilities aren't too delicate, take a look over at this website that Doug Cooper put up sometime during the Clinton administration.
It's a large collection of Thai bumper stickers. Most are transcribed and many are translated. Some of the translations are wrong, and you have to download a special font to see the phonetic Thai, but it's still a nice collection. The content is not the type of Thai you'll want to use in polite company, if ever. Be warned.
Here's one that's suggestive but relatively mild:
"(I teach) wedding night lessons"
And another nice bit of wordplay:
(a play on วัตถุไวไฟ "flammable material", seen on gas tankers, etc.)
Another popular one I've seen so many times now that I began wondering if I'd gone colorblind. The gimmick is sticker lettering on the rear of the car saying รถคันนี้สี(X) "this car is (color), filling in the blank with any color that the car isn't. The lettering is often the color the car purports to be, but not always.
A typical example: a pink taxi might have lettered on its bumper: รถคันนี้สีเขียว "this car is green". I looked around on some Thai message boards, and many people claim it's done to แก้เคล็ด, as Thais say -- to ward off bad luck. I'm sure plenty do it to be ironic or trendy.
One more for the road, from the just-keep-telling-yourself-that department:
"Real men have a paunch."