Warm up while training the head to come up. Because the receiver is always somewhere new, players have to scan for their number before the ball arrives, then move again the moment they have played it.
Mark a 30x30m grid and give every player a number. The ball goes in numerical order and back to one. Everyone jogs continuously inside the grid, and the rule that makes the exercise work is that nobody may stand still, not even for a moment. Six players is the maximum for one ball; with a bigger group run two grids side by side so everyone gets touches. Start at a jog with two touches, then build the tempo. If the group loses the order, stop and re-number rather than letting it drift.
Because the receiver is somewhere new every time, the search is real. In a fixed passing shape a player learns the geometry within a minute and stops looking, since he knows where his target will be standing before the ball reaches him. Numbering the sequence and letting everyone move breaks that, and it means the head comes up because the exercise cannot be completed otherwise.
That is a better mechanism than instruction. A coach shouting at seven-year-olds to look up gets compliance for about thirty seconds. A drill in which a player genuinely cannot find his target without looking produces the same behaviour continuously and without anybody being told.
The timing point is the one worth coaching explicitly. Find your number before the ball reaches you, not after. Players default to receiving first and then beginning the search, which produces a heavy touch, a long pause and a queue of teammates waiting. The search happens while the previous pass is still travelling.
Moving immediately after passing is the other half, and it is what keeps the grid genuinely unpredictable. A player who passes and admires it becomes a stationary landmark, and the exercise slowly reverts to the fixed shape it was designed to avoid.
Keep the body open so the grid is visible rather than just the ball. This is a warm-up, so the passing quality bar is modest and the tempo should build gradually, but the habit being trained here is the same one that separates good midfielders from busy ones.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.