How can I more consistently pop enemies into the air as a soldier?

While it won't provide a full and technical explanation about how to pop enemies into the air, this video shows (in a pretty accurate way) how to do so, using the rocket launcher:

The technique is basically to aim at the enemie's feet. The lighter the enemy is, the easier he'll fly away (you know who I'm talking about). And if you are lucky enough and hit the enemy while he's jumping, he will even fly higher.

Also, as for many other games, watching replays might help to catch the technique. There's a bunch of fragmovies and replays where you'll see the soldier in action, helping you to understand how to improve your soldier's air-popping skill. I'd recommand you that one and this one where this particular technique takes a good part (among others).

There are also videos about the Reserve Shooter's gameplay.

You may try using the Liberty Launcher. The increased projectile speed should assist you in landing the rocket at their position, especially if they are unsuspecting.

This tactic works primarily for medium classes such as other Soldiers, Demomen, and the Pyro, as lighter classes tend to just explode and the Heavy doesn't move very far. Therefore, unless you are out-of-rockets, its best to just place a rocket where they are going to land as this will kill almost anybody who survived a rocket direct enough to launch them in the air in the first place.