I would suggest that you humbly inquire whether they have done it already,
or humbly state that it hasn't been done yet.
So if your boss forgot to reply to an email from last week, you could say something like,
"Have you already replied to (the) email?"
"It seems that the client hasn't received a response yet."
(be sure to trail off with a long pause and make your face look real worried...)
- To avoid being rude, you should try to point out that something hasn't happened yet and avoid "assuming" that they forgot ;)
Also, for bonus points try to look and ask them if you can do the thing they forgot.
"May I reply to the email from the client?"
(this is more effective if you appear to be out of breath...)