Quote Originally Posted by Opie Yutts
1) Have everything possible outside your grow area. Ballasts and fans make heat.
2) Pull your air over your light instead of pushing. If your light isn't completely sealed the fan will push hot air out into your grow instead of sucking hot air out of your grow and pushing it outside.
3) Have a dedicated intake and exhaust just for the light and another for the grow.
4) Intake close to the floor and exhaust near the ceiling, since warm air rises.
5) Get rid of as many bends, elbows or T's as possible, and get rid of as much of that cheapo coiled dryer ducting as possible. Both drastically reduce air flow. You should be using real duct, available in 4 foot lengths at most home improvement stores. The only time you should be using the flexible stuff is when the ducts are attached to something that might need to be moved, like a light.
6) ...cheaper fans do not move the CFMs that they are advertised to. I've heard of one good fan outsucking two of the cheaper ones with supposed same CFMs. I like CanFan and Vortex.
^^good list from Opie. Cheaper fans are less tolerant of increased resistance from shit like bends in the line and turbulence. I also like CanFan.

One thing that hasn't been mentioned is the longer your run of exposed ducting in the space, the more surface area there is to RETURN the heat to the room through the thin uninsulated walls of the flexiduct.

Short, straight ductwork, suck don't blow, make sure the ROOM is exhausting too (if you only have one fan, leave the 'intake' end of the cooltube open to the room to serve as exhaust port too), insulate ductwork if you still have a lot of it exposed.