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,
「もうメールにお返事されましたか」
"Have you already replied to (the) email?"
or
「先方からのメールにはまだお返事頂いていないそうです。」
"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.
「先方からのメールにお返事させて頂いてもよろしいでしょうか」
"May I reply to the email from the client?"
(this is more effective if you appear to be out of breath...)