The key is that you have to *do something* when returning any long serve or you give the opponent a big opening. And that usually means attacking it. Against a long serve to the backhand, that usually means either backhand looping or hitting/punching a strong backhand. A player with good footwork may step around and loop with the forehand, but most can't do that on a regular basis unless they are very fast or they anticipate the serve. If the serve is fast, you can use the speed against the opponent with a punch block. If you have trouble attacking the serve, try shortening your stroke.
You want to place the ball, usually wide to the corners, or (if the opponent isn't looking to attack with the forehand) a strong shot to the elbow. Shots to the middle backhand or middle forehand put little pressure on the opponent, and are often ripped.
You want to hide the direction. For example, if you aim your backhand crosscourt to the wide backhand, then at the last second change and go to the wide forehand, you can catch an opponent off guard. If you aim to the wide forehand, many opponents will move to cover that, and then you can do a simple return to the backhand.
You want depth. Even a weak topspin ball that goes deep can be effective if it either has topspin or is to a wide angle. (However, you don't want to rely on this - a good player might still tee off on this.) Against some players who hang back to counterloop, a shorter, softer, spinnier topspin return is more effective, but don't overdo it or they'll get used to it.
A sudden chop, chop block, or sidespin block can also be effective, but only if you can control it, and usually only as a variation. If you can deaden the ball with a chop block or sidespin block, many opponents will have great difficulty. If the serve has sidespin, try sidespin blocking it back, using the opponent's own spin against him. (Go with the spin, not against it, i.e. against a forehand pendulum serve, your racket should go right to left for a backhand sidespin block.)
Lastly, variation is important. If your opponent knows what you are going to do, things get pretty easy for him. Even if you are going to loop all deep serves (as most advanced players should), you should vary the placement, depth, speed, and spin, and throw in sidespin loops as well.