From M. Baswell:
If you can do propellers on both hands (no, I don't mean at the same time, though that certainly qualifies as a harder trick!) and you can do helicopters...
Start in a helicopter spin, single-sticking (i.e., tapping the baton with just one handstick at a time). From there, catch the baton near the middle, and do a single 360-degree propeller-style spin before bringing it back. Try to keep the plane of the baton constant, so that the propeller spin is nearly flat. This is a neat thing in itself, but it's really only the first step to a much neater thing.
Next, try doing flat half-flips with one hand. Imagine the motion of normal one-handed half flip - forehand, backhand, etc. Starting from a helicopter spin, try to do the same thing, only with the baton still mostly horizontal. A goodly amount of forwards-backwards motion helps to keep the stick in that horizontal plane as you do this; pretend you're fencing, thrusting at and parrying the baton. If you really get into it, it looks very flashy.
Once you can do both of those tricks consistently, try to keep the helicopter-propeller spin going without returning immediately to the helicopter. Basically, inscribe a cone with your handstick, always keeping it under the baton. You'll need a lot of wrist action and practice to make it more than two or three complete circles. You can also do this overhead, which is a good way to disturb people who don't know what you're doing and can't understand why the funny-looking baton is staying on the handstick.
Now, from that helicopter-propeller spin and those half-flips, you're ready for the trick which I'm really trying to teach you. Try doing those half-flips while keeping the handstick in contact with the baton as much as possible. In other words, try to always keep the baton rolling on the handstick. In order to do this, you'll end up pushing the tip of the handstick up, then down, then up, then down. Exaggerate that motion. Get your entire arm into it, so that the baton is spinning first above your forearm, then below it. Eventually you'll be inscribing two cones with your handstick; a cone extending upward as the baton moves above your forearm, then, as you push the handstick down and move your arm over the spinning baton, another cone extending downward. This motion is uninterrupted, and once you've gotten it down the baton will be in constant contact with the handstick.
This move is called the curl. Once you've got it, you can use to carry the baton all around your body, such as behind the back in a smooth, flat spin, or between your legs, while keeping both feet on the ground (none of this kick-up-a-leg crap!). I call these tricks "curl-and-carry" moves, because they're just the curl, only with a bit of gratuitous flash.