When receiving, many players are either overly aggressive or too passive. It's important to find the middle range. However, it is even more important to understand that it is consistency, placement and variation that are most important.
Flip kills and loop kills are exciting ways to return serves. However, they are also quick ways to lose the point via missing. Always remember that all you have to do is break even on your opponent's serve, and you'll probably win on your serve. So you don't need to hit a winner off the serve. Just return it in such a way that your opponent can't hit a winner - which normally means catching him at least slightly off-guard. To do this takes good placement, variation, and hiding the direction and shot selection until the last second.
If you don't place the ball well, your opponent may jump all over your return. Few players can cover the entire table with a strong third-ball attack, especially if you don't telegraph your direction early in the shot, so it's important to figure out what part of the table your opponent will have the most trouble with, and go there. A well-placed passive return is often more effective than a strong return hit right at an opponent's strength.
Without good variation, of course, your returns are predictable. Mix in loops, flips, pushes, drives, and do them at different speeds, spins, depths, as well as varying the placement. Aim one way, then at the last second go the other way so your opponent can never know where you are going until you contact the ball. If your opponent looks likes he's looking for a push return, give him anything but that.
What is your primary job in returning serve? To mess your opponent up! Go to it.