Maxims and opposites:

You can't teach an old dog new tricks. - You're never too old to learn.

Look before you leap. - He who hesitates is lost.

A penny saved is a penny earned. - Penny wise, pound foolish.

But as for favorites, ahem. The ones below aren't time-honored, but they certainly Sound maxim-ish!

In the beginning, there was order. Random order.

You can't tell which way the train's gone by looking at the tracks.

Tsyganka, who finds that a bird in the hand is usually messy