Vary the degree of spin. By adding more or less spin to each ball, you will be able to learn to better control the ball and you will make it more difficult for your opponent to comfortably block your loop. If you add the no-spin variation, be careful not to loop with too much speed. The more topspin that you add to your loop, the more potential for speed that you can have while still maintaining consistency.
Vary your distance from the ping-pong table. Against some blockers, you need to stay quite close to the table in order to loop. Against some counterattacking/smashing players, you need to give yourself a bit more distance from the table in order to loop. Regardless of who you are up against, it is critical that you be able to smoothly move in-and-out away from the table. If you have merely been practicing side-to-side footwork, it is time for you to add the in-and-out variations to your practices.
Vary your ball placement. Improving your ball placement is one of the fastest ways to improve your table tennis game. Against those players who are comfortable with both backhand and forehand, you should often target the center transition point. Against those players who are trying to play full-table forehand or full-table backhand, then you need to work the angles in order to make them play with their weak side. When practicing ball placement, try not to be too self-focused on your stroke, but instead be focused more on your opponent – his strengths and his weaknesses.
Vary the type of spin. By adding various amounts of sidespin to the ball, you can make it much more difficult for your opponent to block or counterloop. Practice adding slight amounts of sidespin to your topspin loop. Once you become consistent at that skill, next try to add more sidespin or even sidespin-backspin. The more topspin you apply, the more speed that you can add to your loop. If you loop with pure sidespin or sidespin backspin, make sure that you focus on control, not power.