Reduce testing while writing code
Very often it happens that more time goes into testing than writing code. Particularly when ajax is involved.
For example, imagine this scenario.. you are working on an online store and you want to allow a user to remove items from the cart using ajax. ie. when the 'remove from cart' button is clicked the item must disappear from the cart...
I have found this approach extremely effective when some irreversible effect is involved, mostly deleting something or in other cases such as adding an email address (or any other field) in a table where the email column accepts only unique values.