As a lawyer, sometimes your communications require prompt responses. But commonly used language can only go so far when it comes to expressing urgency. Milder expressions like “at your earliest convenience” or “at your soonest” are fine in many cases. Slightly more aggressive approaches such as “please send immediately” or “need ASAP” are also often used when deadlines are looming.  When it really gets down to the wire though, I like to bust out “send at your promptliest immediance”.