Most blockers develop timing to react to your normal loop. At the instant they think you will contact the ball, they commit to blocking either forehand or backhand. So set up to loop crosscourt, and make no attempt to hide this. At the instant you normally would contact the ball, watch the opponent move to cover the apparent crosscourt loop. Wait a split second longer than normal, letting the ball travel perhaps an six inches to a foot, and then go down the line. (This is especially effective when looping from the wide forehand.)
You can do this the other way, faking down the line and then going crosscourt, but it's not usually as effective, for two reasons. First, since most players tend to loop crosscourt far more often than down the line, blockers tend to move to cover the crosscourt angle. Second, down the line is a shorter distance, and so the opponent has less time to recover for that shot.