This is probably really stupid, but, why don't they just take off and land with the banner already attached?
Well, some operators did used to do that with smaller banners and bigger planes (but I'm talking back in the 40's here). It is incredibly hard on the banner to be dragged across the ground, puts a lot of wear on it, and it's a helluva lot of drag to takeoff with. I don't even think it would be possible to takeoff with a large billboard in tow.
It never fails either, every operator I've seen has a slightly differant pickup technique (hell, most pilots I've seen all pickup differantly too). We used a slightly shallower approach from 200-300' (depending on the plane) and aimed for 80 mph through the poles (ours were 5' high and 10' apart at home, 3' high and 4' apart for temporary poles away from home). We rolled the power on about 3 seconds before you rotated, and hauled back to between a 35 and 70 degree climbout depending on the plane, wind, banner and pilot. We'd kick full rudder on the climbout to see if we caught anything (we didn't have radios). Pushover would be at around 250', if you did it right you'd be leveling off right before the stall, just as the banner started to come off the ground, if you were fast you could pull the power back a touch as you lowered the nose, the transitioned to a climb. I never felt a tug after my first month, sometimes the ground crew would tell me later that they'd see the banner go up, the pause and settle down a few feet, then go back up again, as I'd be a little TOO slow on the top.
Other fun things at our company, was after pickup, we then had to make a tight climbing turn, and look back to proofread the banner (and find out where it went).
Other companies I've seen use a normal 4 or 5 degree approach with power at 90 mph, followed by a sharp pull, transitioning directly to a steep but sustainable climbout.
All in all, some of the most fun I've ever had flying. Best was when there was a cop car in our drop zone (don't know what he was doing there, but it looked like a target to me) I missed though; the lead pole hit about 2' behind his bumper. He did get the message though and was gone when I landed
