Whenever I use a hand saw, I find I keep subconsciously curving the cut, no matter how much I consciously try to compensate.
In this example in photos, to get this cut straight I kept rotating the pole, but from the messy spiral shape, you can see what was happening.
What am I doing wrong?
This is why miter boxes exist. They keep your workpiece and saw in reasonably stable alignment. Some clamps to keep things still make nice cuts almost effortless.
Remember: Let the saw do the cutting. Long, light strokes. Don't force it.
You can make your own fairly easily with a few boards (ideally hardwood) in a U configuration and a square cut. To facilitate that, clamp some temporary guides to the miter box for the initial cut--a miter box for the miter box, if you will.
The same way you get to Carnegie Hall - practice, practice, practice!
Of course, there are some tricks that you can use to make it easier.
Draw lines on 3 surfaces. The first line is on the top surface where you begin your cut, the other two lines are on the adjacent surfaces. Assuming these 3 lines all line up, you can stop every half-dozen strokes or so to check that the saw is tracking along the two lines on opposite faces and make adjustments before you get far off track. You'll also have lines to follow as you go through the wood to help you stay on track.
Ensure that you have correct positioning when you're cutting. You want your cutting arm to be able to move freely past your body - that means the cut line should be to the side of where you're standing, not in front of you.
Use a guide to help you stay on track. Something like a Speed Square™ (or one of the many knock-offs of that brand name). You can line it up on the top face of your cut and use it as a very visual line to follow if you're having issues following a simple pencil line.
Make sure your saw is in good condition. If there is any damage to the saw, it will make it difficult for it to track straight. Obvious bends or kinks are one thing, but there may be a subtle bow in it, for example if it was stored laying flat with something under it (laid flat in a drawer). Or, even a single tooth being knocked out of proper alignment could be causing it to wander off track - this could be caused by the saw being dropped, or something dropping onto the saw, and hitting a tooth.
Finally, while you're getting the practice and you aren't getting the results you really desire, cut your boards just a bit longer than you really need them with the knowledge that you'll need to do a bit of sanding to get them squared up and to their final length. This way you can cut and be productive without wasting material because you've cut a piece too short.
A properly sharpened saw.
A clear line.
Practice. Less than 20 years, but actually practicing with scrap lumber is very beneficial.
For 90 degree cuts, it also helps if your saw is clean and shiny enough to reflect the wood so you can see if the reflection is straight, or crooked - crooked indicates that the saw is not perpendicular to the wood.
One thing I've found immensely helpful is a speed square (this one is metal, but there's some PVC models that are really cheap)
They have a "T" top, so you can set it parallel to the side of your board and draw a straight perpendicular line against it. On a 4x4, I'd draw one on the top and two sides. Then do your best to take the line with your saw.
The issue with this sort of cut is that the bevel and the miter are not perpendicular to the workpiece and not perpendicular to each other.
Use a jig. You can build one, and place the workpiece in it, or you can build a jig around your workpiece by clamping or screwing on some guiding lumber. The jig separates two key steps in sawing straight: a) the patience and precision required for alignment, and b) the steady perseverance required for the sawing.
Alternatively, clearly mark the cutting outline. Wrap the workpiece with painters' tape along the desired cut line. Careful though: all tape has elasticity, so while wrapping don't pull the tape hard, as it will warp. You need a colour that clearly stands out.
In cases like this I use a piece of newspaper to wrap it first, and mark the edge of the paper for where the tape should go. Mathematically, if the paper edge is tight and aligned at the seam where it wraps around, it is perpendicular to the axis of the workpiece.
This also means that the miter and bevel are perpendicular, and they are both zero.
Depending on which side you are cutting into, the marker is a bevel for one side and a miter for another side. Keep a sharp eye on both markers at the same time, to get a straight and perpendicular cut.
Here's what I've learned over the years:
It takes some time to master, but if you follow these three principles, you should see improvements straight-away
For general purpose use like this you should be using a standard crosscut handsaw, with 8 to 12 teeth per inch.
It looks like your saw has a bent tooth or has hit a nail. The saw's teeth have a "set" to them, and sometimes the teeth on one side can be damaged, making a saw cut a curve. The picture above shows a lot of wander for such a small distance. If your saw is damaged, you'll have to repair or replace it to get a good straight cut. Damage is:
+1 on all speed square advice. mark the top of the board, then mark an adjacent side, then, from the top again, mark the other side.
Start on the far corner. Keep you elbow, wrist, and saw in a line. don't let your elbow wander around.
It won't take long before you can split a pencil line.
The technique I use was taught me by an old craftsman in the 1960s. It has served me well. There are several tricks to it.
Mark out the cut on all four sides with a set square and soft pencil. The fourth line closes the marking and gives you a cross-check that you have been accurate and not let the set square slip.
Get your shoulder, elbow, wrist and saw blade all aligned in a vertical plane. Never let your arm twist out of the plane of the saw.
Keep your shoulder still while you saw. Make light cuts; let the saw do the work, not you. Take your time, don't try to rush the job.
Start at one end, usually the far one, work the cut gently back along the line. When you reach the other end, flatten the saw out so it runs parallel to the top face.
Don't expect miracles, it comes with practice. But it does come.
Existing answers have great tips. Here are two more small ones that can make a big difference.
A couple of things I didn't see mentioned. Keep one eye on the saw blade - it's easy to line up back to front of a cut, but unless the blade is vertical, your problems start. So watch the blade stays vertical.
A tenon (back) saw has a more rigid blade, so will wander less. But they're usually short, and a crosscut (ripcut) saw will be used. Once into the cut, keep as much blade depth as possible. The narrow end will tend to wander more than the deep end closest to the handle.
6+ months down the line, and for posterity I wanted to add what has (mostly) worked for me, in addition to the advice of, "practice, practice, practice!"
Cut outside the line and then use a belt power sander to square it back to the line, that way you won't have to worry about it being straight.
I do it regularly.
