I use my mouse to fly. Thumb button (Logitech Mouseman) to descend, right button to ascend. In order to do it you need to set C and spacebar as the primary jump and crouch keys while setting MOUSE2 and MOUSE3 as secondary.As for my other keys:
Mousewheel: change weapon
] and [: zoom in and zoom out, respectively
End: Reload
MOUSE1: fire
MOUSE2: jump/ascend
MOUSE3: crouch/descend
Forward Slash: Use
ENTER: Secondary Fire
SHIFT: Walk
Up and Down: Forward and Backward
Left and Right: strafe L/R.
I find this config places all important keys near my hand, and since I hardly ever snipe, I don't use the zoom function much.