I use a few tricks that work well for me.
First, the tried-and-true magic marker trick is more important with recurves than anything else. Recurves are a bit more difficult to sharpen, so it's especially important to have a visual indicator of your progress. At least, it is for me.
Second, to keep the angle constant, I want the part of the edge that's touching the stone to always be perfectly parallel to the ground, and perpendicular to the stone. To do that, you need to move the handle up and down as you pull the blade along the stone. It sounds awkward, but it becomes second nature quickly. I start at the very back of the recurve nearest the handle. Because that part is sloping down slightly, I raise the handle up slightly, to make the edge parallel to the ground. As I pull the knife back towards me, I'm lowering the handle until it's perfectly parallel to the ground when I get to the peak, then start lowering it as I continue. Parallel to the ground again when I get to the bottom of the belly, then up as I get to the tip.
One last trick that I've just begun using, that works fantastic for me, but I suggest you don't think about this until you get the basic stuff right. I set the stones up as if I will be working with the flats. But for most of the stroke, I cant the handle ever so slightly towards the back of the stone, so the blade comes off the face of the stone and only contacts the corner (being careful to keep the spine straight up and down no matter what, of course). As I near the end of the stroke and get to the tip, I cant the handle back, and I finish up the tip on the flat of the stone, so it will sharpen up well but won't slip of the corner and get rounded.
All of these strange movements become second nature easily. I started doing the last trick I describe above when I sharpened my Apogee last week. By the time I was done, the movement was in muscle memory and I didn't have to think about it anymore. When you think about it, the overall movement is quite complex -- I'm raising and lowering the handle as the blade goes through its curves, and also canting the handle and uncanting as I get to the tip. It sounds way harder than it is. Get the handle movement down first. You may not find the canting trick necessary, but you might try it, it's great for keeping the tip up to snuff.
Joe