Don't be a Trump

Don't be a bad looser and stop throwing your toys out of the pram.

Donald! You lost the competition so don't be a Trump and man up!

don't be a wally

Don't make yourself seem, or act like you are less intelligent than you actually are.
The actions of someone appearing to be like a Wally, may be either unintentional or on purpose.
Being likened to a Wally, can happen either affectionately, in a sarcastic and demeaning way, or as encouragement not to be stupid.

"Hey mate, don't be a Wally."
"Ha-ha.... Don't be a Wally."
"That's amazing, but don't be a Wally."

fon-don't

A tragic fondue error. While at a fondue party with friends, not following fondue protocol can result in a fon-don't.

Keeley went overboard lighting the flame and started a huge fire under the fondue pot. She committed a major fon-don't.

Somehow, Chuck got the sauce for his chicken on his shoulder. He was wearing a fon-don't on his sleeve.

Troy tried loading too many items on his fondue fork and they fell off in the oil while cooking. Leaving food in the fondue pot is a fon-don't.

Don't be a Simon

Used in the early days of American Idol when the world was new to the competition and Simon Cowell was the biggest douche bag on the planet. This phrase can be used the same way as Don't be a dick!

Dad: Thanks for making this wonderful breakfast kiddo.

Son: You're welcome dad.

Mom: ...I don't know..the pancakes were a little dry and the eggs were undercooked..and don't get me started about the-

Dad: Dammit babe, Don't be a Simon.

Don't ask Don't tell

Some policy that said gays can't be in the military if they were public about it

don't ask don't tell bill clinton go to hell

don't panic

The most useful phrase in the english language. Popularised by Douglas Adams in The Hitchhikes Guild to the Galaxy - the best book ever written.

Somone wrote Don't Panic on the cover of my calculator. It gets me through exams.

don't be a stranger

Said to a person or workmate when they aren't going to be around anymore but not going to be far removed in a geographical sense. For example, if they are changing jobs but remaining in the same town or city.

"You were a pain in the ass to work with but I'm sad that you're leaving for some reason. Don't be a stranger, will you?"