I have two drills/ideas that I put into action whenever my players start to drift. 

1) Set up a normal 4 lane passing drill and have the players pass the ball down the line as usual. After a few minutes start to add in cones/ tackle bags in front of the first 3 players to act as an obstacle. Instruct your players to start to step inside of the obstacle before releasing the pass. In other words fixing the player and preventing the players from drifting out. The next progression would be to add in 2/3 defenders and your players will notice the benefit of stepping in (not drifting out) in fixing a defender and creating more space!

2) In normal game play practice or run throughs create a new rule. This rule should be that every time one player starts to drift, one of the outside players must angle their run in/ run a hard line. I normally make the players do push ups if they don't angle the run in or drift in attack. 

Hope that helped!