You should continue to look forward unless you plan on stopping.
The idea is to keep looking where you will be. When you direct your attention at something that is coming nearer to you, you lose sight of where you will be after it.
Often times, you can ride/slide through considerable amounts of sand/gravel/dead bunnies if you have your head up looking where you want to go. If you however, are looking at said objects, you will most assuredly be taking a closer look at it momentarily.
Concern yourself with things that you can clearly make out in your view ahead. A small rock on the road may slide under your radar as a blip ("What was that?") and shouldn't be given much thought. A big rock you'll definitely show up clearly on the radar and that's when you take action.
The most confusing part of people telling you to look ahead is just how far ahead. On a long straight road, you can see very far ahead, but you don't want to be looking at the horizon. You should focusing on the area just ahead of your stopping distance but no closer than about 1.5s.
For example, at 65mph you may need to look 3 or 4 seconds ahead. At 5mph, you could be looking right in front of your tire because bikes stop so fast, but you want to be looking at least 1.5s in front of where you're going.
The most confusing area is in the corners. I hear people say that you should be looking into the turn. What i think this ends up between the ears is that people think they should be looking as far around a corner as possible. You should be looking where you want to go.
Let me explain. I've seen far too many people get up to a corner and start cranking their heads. What they then do is cut the apex too early and swing wide on the far end. They are so focused on looking ahead that they lose track of where their bike is going. As you get to a corner, you should be looking at where your turn in and apex are, but, here's the trick, don't linger at any given point. Your bike is still moving forward and so should where you're looking. Your vision should "roll over" these key points as you go through a turn.
Here's an exercise you may want to try. Find a road with some bends in it. Nothing too terribly twisty. Then visually cut your lane in half. Now ride the left half (closest to the middle) through the bends. Don't lean the bike over either the middle line, nor your imaginary line. I will often see newer riders try this and they can't stay in the half lane. Going straight in a half lane is easy. In turns, it forces you to focus on where your bike is. Your bike is where you told it to go when you were looking ahead.
Anyways, i'm rambling now. Just remember to never drive faster than you can see forward and stop.