Originally Posted by Bigcorona
Try to keep a couple inches of water on top of the grain bed.
Assuming you have a well designed manifold, I think keeping the grain bed flooded is the most important thing when fly sparging. By keeping the grain bed flooded, it is harder to get channeling through the grain bed as all of the grain is in constant contact with sparge water. If you cannot maintain a couple of inches of water on the top, slow down the speed you are draining. Slow and steady is the key.
Trying to maintain the balance between how fast I was draining and how fast I was sparging was the reason why I switched to batch sparging. It was faster and easier to just dump in all of the sparge water, stir the grain, then drain a couple of minutes later. For me it was just as efficient, possibly because I made the switch before I really got good at fly sparging.