One of the more noticeable things among using the larger lenses is "zoom creep."

Zoom Creep is a lens' tendency to slowly (or rapidly) extend or shorten, depending on the downward or upward tilt of the camera. It absolutely necessitates the use of two hands while using the lens and is problematic.

Several people have suggested different methods in dealing with this problem, my favorite being the use of a piece (or pieces) of "gaffer's tap" along the barrel of the lens, to "lock it" at the zoom length you have chosen for your shot. Gaffer's tape rarely leaves residue, unlike masking tape and such. Get a roll.

One of my favorite lenses, the Tokina AT-X 840 80-400mm f/4.5-5.6 suffers from this problem, so I do have some experience dealing with it. It is made entirely from metal and weighs a goodly amount. There are many other lenses that also have it, but in different degrees. I have found the more you use your zoom lens, the more you increase its chance of developing "zoom creep." Call it an "old age" problem. A handy idea would be for the manufacturer's to design an adjustable knurled screw that could provide additional pressure along the lens barrel, to retard this tendency. Cripes, I may just machine it myself. Heck, it's my lens.

Anyway ... for the plastic lens barrels ... stick with the tape (pun intended )