If you can’t practice perfectly – and who can, when you’re just learning how to do something? – then there are two things you need to do. First, break down what you’re doing into easier, more fundamental tasks. And second, differentiate between practice and experimentation.
Break it down: If you can’t saw perfectly to a line, then work on your saw technique independently of cutting to a line. Having trouble with that? You can break it down even further. Make sure your grip on the saw is relaxed (like you’re holding a baby bird), that your wrist is straight, and that your forearm lines up perfectly with the back of the saw.
Make sure your stance is good, and your body is oriented correctly to allow your elbow to swing just past your hip. And practice the sawing motion with the power coming mostly from your shoulder. The saw stroke should be relaxed. It should sound relaxed. Long smooth strokes, with very little downward pressure. Let the saw do the cutting.
Once this is feels natural and you can get this feeling pretty much every time you pick up a saw (it shouldn’t take all that long), start in on the lines. But you can practice cutting to a line separately, too. Go to the bandsaw and practice there. It’s a great way to get comfortable with cutting exactly where you want to cut without having to think about all of the other body stuff. Then start to work on putting the saw technique and the lines together.
Try not to think about too many things when you’re learning something new; you can’t do it. Some of what you’re doing has to be turned into ‘muscle memory.’ The neural pathways have to be well established so your body can do the task right without expending much conscious thought. And that will allow you to think about the next element of the task; in this case, cutting to the line.
Practice vs. experimentation: You also need to learn to differentiate practice and experimentation. You’ll have to experiment with things – try out different grips, moving your body an inch this way or that, using more or less pressure. Pay careful attention to how things feel, and to what works and what doesn’t. You’re going to have to learn through experience just how much pressure to apply (not much), how to stand, how to get the saw started (even less pressure). so that you get the results you’re looking for. The key is to know what you’re looking for, so you can tell the difference.
More thoughts on this can be found in my new book, The Foundations of Better Woodworking. You can pre-order it here: http://www.shopwoodworking.com/foundations-of-good-woodworking-w8062