It takes practice, but it's worth it.

Learning to saw to your line and saw squarely takes a well set up saw, some technique, and a lot of practice but it is absolutely worth while. Once you can saw well you save a lot of time cleaning up and getting down to your line. If you feel like it's impossible to do just keep working at it, the breakthrough will come in time. It's taken me a lot of sawing to confidently do a rip cut on the line when it matters.

Laying it on the line

First things first, the saw itself must be in shape to cut straight. Look at the teeth, are they all the same size? If they are not the saw will cut off the line toward the side with the longer teeth. Is the saw bent or kinked? This can have an adverse effect on the sawing. Are the teeth sharp? You can check by laying the tooth line across the palm of your hand, do the teeth catch readily against your skin? If not the saw is too dull. Forcing a dull saw makes it hard to saw accurately. If you are unsure about the saw itself it's a good idea to have it professionally set and sharpened, then you'll be sure of the saw and can work on your technique.

Going the distance

A well set up hand saw or back saw wants to cut straight and square. It's usually the sawyer that makes it do otherwise. Grasp the saw with a three finger grip pointing your index finger along the side of the handle. Hold the saw but don't grip it too tightly. I've heard a saying that you should hold it like you would hold a mouse you are trying to keep from escaping but don't want to harm, seems reasonable to me. With this light grip the saw will point down plum of its own accord.

Take the time to get a good start in the cut. Make sure the start of your cut is square and on the line, starting out well is easier than trying to correct later and a good start guides the saw straight as you saw further. Take light strokes to start and wait to apply more power until the cut is well established. Once the cut is started well use as much of the length of the saw as your arm allows.

All square, ready for cleanup

Now that you've sawn straight and square cleaning up the saw marks with a lightly set jack or try plane should only take a few quick passes.

As I said before it has taken me a lot of practice to get here but there's nothing like that feeling of completing your first square 8 foot rip right on the line. You'll feel confident to try anything.