I actually have extreme problems with drawing curves as well. I hate them, which is one of the reasons I use vectors... When I have to deal with natural curves on the other hand, there are a few things I do. But always remember the golden rule; the eraser is your friend.



First is the exact tool. If you can use tools that don't have pressure sensitivity (fade away slightly near the ends), then erasing part of the curve that doesn't look right and starting from there, is best.



One thing I'm lucky for is the Stabalizer option in SAI. It actually sets the way the program interprets my tablet's input, smoothing out lines for me. Without this, my lines would look janky as all hell.



Another thing to do is practice relaxing your arm when drawing. I used to have a very stiff arm when drawing and I made short, sudden lines that didn't go far. Now with a pen and paper, that wasn't so bad, but with a computer screen that could zoom in on, I needed to learn how to draw longer lines. I can't exactly give you a tutorial in that, but one bit of advice I found helped me a lot, is to be confident in your stroke. Don't hesitate on a smooth line. Like, actually mentally reassure yourself the line will come out nicely and relax into the curve, bending your whole arm into the mouse or tablet.



Also, Ctrl+Z, the holy Undo button. Its a reflex to use it and draw the line again. If my hand is not hovering over the keys when I'm drawing, it feels off to me. Use that and redraw until the line comes out.



I guess,... Try to take curves one at a time. For example, if the line bends in the opposite direction halfway through the line, draw the first curve and make sure that looks good, then draw the second. I've spent way to much time in frustration doing just that.



Finally, I guess zooming out could be an option. It means you have to draw a smaller distance on the screen and it shrinks the space for error.



But, yeah. I'm in that boat too. I think everyone draws poor curves, you just hide it with tools and tricks. I hope that helped.