Tell the person he should definitely keep a journal of some sort, something that he can have with him at work, home, and in the car. Tell him to write down any important events as they are happening. Write everything down.
Tell the person to keep a to-do list of sorts. Every day, the person can check to make sure he's done everything. Also, he can write down what he needs to do as he becomes aware of its need to be done.
Tell the person to leave sticky notes places. If he's almost out of milk, he would attach a sticky to his coat or something. That way, when he goes out the door, he will be reminded.
These can be difficult to start because it involves remembering, but they should be helpful once adopted.