Since I have it in front of me, some additional data points from Hieronymus' Hops book (pp 215-6):
Stone dry hops between 1/3 and 1.25 lbs per barrel (0.17 oz to 0.65 oz per gallon, 0.86 to about 3 oz per 5 gallons)
Lagunitas between 0.5 lbs and 1.5 lbs per barrel (1.3 and 3.9 oz per 5 gallons)
Brewers all seem to agree that more is not necessarily better. Marble Brewery (p. 28) got better flavor and aroma after they decreased their hopping schedule, and there is also a point of diminishing returns on the upper end: Rock Bottom saw very little difference between 1/2 lb and 1 lb per barrel (1.3 oz and 2.6 oz per 5 gallons), New Belgium found their ceiling to be 0.9 lbs per barrel (2.3 oz per 5 gallons). Then again, I would bet that the ceiling is different when you're working on a homebrew scale.
Vinnie Cilurzo of Russian River is a believer in multiple dry hopping (put half in for three days, remove them and put other half in for three days) while Jeremy Marshall of Lagunitas Brewing thinks multiple additions is a waste of time. Firestone Walker does two additions at about 1 lb per barrel for Union Jack IPA, but strongly believes that short contact time is key: no more than three days. On the other hand, Dogfish Head's Indian Brown Ale dry hops with only 1/2 lb per barrel but keeps it in for 21 days.
One of the big points made in that book is that though hop bittering is pretty well understood, nobody really knows what is going on with aroma. It depends heavily upon hop variety and, for now at least, there is no easy answer as to how to get more/better/etc. aroma in beer. If you find a method that really works for you, remember what hops you used it for because it just may not work for you with a different kind.