Without seeing the car and doing some tests I would ave to say, first check the belts are tight and not slipping, but more that likely, the alternator is on the way out, voltage regulator and brushes possibly bad. It would be worth having it tested it and reconditioned.

I had a similar issue on an old rover coupe, which believe it or not, had a Honda engine! Anyway it was doing what you described. Starts off, the battery light would flash not too Often, Then gets more frequent over the weeks/months then eventually it will just stop working. I had the alternator reconditioned and it was fine after that.