Send in your Axioms

I'm looking for media planning axioms - things that are taken as truth or as rules of thumb and have been for years. They're for a piece I'm writing either for the blog or for iMedia. Please submit in comments. Some examples of existing media planning axioms:

"Half the money I spend on advertising is wasted. The trouble is, I do not know which half."

"If you don't have enough budget to be effective, save your money until you do."

"The surest way to overspend on advertising is to not spend enough to do the job properly."

Any such sayings you've come to understand as fundamental truths or rules of thumb within media planning would be greatly appreciated.