1/8 to long , then 1/8 to short..what is the secret to cutting correct size?

Any number of ways, so I'm sure more will follow, depending on what works best for you. More often than not, cutting same lengths that matter, say legs, sides of a box, whatever, I would use stops clamped to the miter or the work surface. Once in a while I want to see exactly where my cut will be on the table saw, so I lay a piece of masking tape across the table, tuck it over the ends, because it does not stick well to a waxed surface, meaning you have to be careful and can't dilly dally, then cut a piece of scrap with the miter, bring it back, and mark the cut edge to the tape. If "free handing" it on the table or miter saw, I find it easier to focus on a single mark, right where the blade will first contact the wood, rather than a line. If you do want a line, minimize the steps to reduce error. Rather than measuring the piece, making a mark, and then laying your speed square to the mark, hold your speed square to the piece, measure to the edge of your speed square, and draw your line. Might sound the same, but it's one less step, so one less place to be off a bit.

When using a portable circular saw, a shop built saw guide will put it on the money every time and make repeat cuts. If X-tream accuracy is needed use your pocket knife rather than a pencil to mark.

If using a miter saw I've adopted the sliding stick method of setting the stop when that accuracy is warranted. One end of the stick touches the saw tooth set in that direction with the opposite end at the stop. If the measurement is between 2 fixed objects the tape measure stays in my pocket and to set the sticks the ends are put on their respective marks. Example: cabinet opening, face frame to face frame distance.

When using the table saw and sliding sticks, a clearance block is set against the fence with the sticks between the block and a blade tooth that's set in that direction. Once again, only if the accuracy demanded is warranted.

Several posts on this thread bring up the important point that much of the time it is important that your cuts be repeatable, not necessarily perfectly accurate. A repeatable cut means that each piece is almost exactly the same length. An accurate cut means that each piece is within a very small tolerance of the desired length. It is MUCH EASIER to make a repeatable cut than an accurate cut, since accuracy requires a sharp line, an accurate measuring device, and a well tuned saw, whereas repeatability simply means you do the same operation each time.

Several posts mentioned using a stop block, this works very well for miter saw and table saw cuts. For rips with a circular saw, you can set a fence against a sharp line. Another method mentioned that I use a lot is to cut multiple pieces stacked together on my miter saw, squaring the ends using a block of wood. Think about alternatives to measuring, measurement with a tape is prone to error due to faulty tape, bad eyesight, fat lines, or poor tape technique. Tage Frid, a famous woodworker, has an entire chapter in one of his books on woodworking devoted to repeatable cuts, and techniques to mark without measuring.