It's much more complicated that this but here's the simple answer.
Ask him when he needs (A) to be done by. Finish right on the day (A) is due. By doing this I am hoping
i) Before I start, something will change and (A) doesn't have to be done
ii) Before I start, I can convince my boss that (A) doesn't need to be done, or the same goal can be acheived some other way
iii) Someone else finishes his task before I can start A, and my boss will just assign A to that person (I know this is sneaky)
But, if I have a task, the task will be done. If the task is higher priority, I will work on it before other low-priority interesting tasks. If the task is not interesting, I will do some other interesting tasks at same or higher priority. But, the task will be done before the task is due.
ETA: Oh yeah, if your boss doesn't ask about some task for 20-30 days doesn't mean that the task need not be done. If your boss has given you a task, you have to finish it. If you feel that he has forgotten about it,
you should clarify. Your boss shouldn't be after you all the time.
[ August 08, 2005: Message edited by: Jayesh Lalwani ]