I try to pin it to my chest/beer gut, that seems to help me
. Your saw has to be cutting good and straight, i find when i hand sharpen several times the cut will start curving right in the cut (silver maples really seem to accentuate that too), so i gotta use a grinder every now and then to get them all the same length again so it'll cut perfectly again. Fighting that a bit on the ground is one thing, doing it while on rope in the tree is another, basically if you gotta run a big saw on the tree make sure it's good before you go try it. You can also cut a bit, just enough to hold the saw in the cut and stop, and then get a good look at it and adjust your aim if needed before sending it, a little bit off at the beginning will often be terrible when it's finished. If I'm gonna have to do bore cuts and stuff I'm not above starting/ doing them with a smaller saw, especially if I'm running the 32" bar on the big saw, that's a nightmare to do on rope. I don't like running the big saw on rope, so I'll try to limit how many cuts i gotta do, and try to keep them super simple and not as dependant on accuracy.
Rather than cutting it perfectly I'll do the magic cut thing, basically a snap cut with a snipe (make sure to finish your cut low so it can't snatch your saw) and I'll even go so far as having someone on the ground trip them with a pull line since that's much easier for them than me pushing on it. If I'm in a lift I'll do the cut, shut the saw off and stow it, and then boom back up to the top to push it, that way you can aim it even better with your push. Rather than blocking it down I'll use a vertical speedline if possible, it's much easier to control a bounce than to catch and lower something that heavy, and you can often take much larger pieces once you stop trying to catch them. You can also move logs around where you want to use as an anchor, drifting a log over a bit is still easier than catching it. Ideally I'll just drop the spar and not need to cut up the larger wood in the tree at all, or free fall them rather than rigging, but sometimes that just isn't possible. As you do it more you'll learn what you can get away with. You can use logs and brush that you've cut already to pad the ground and fence in the drop area too, old tires make great cushions to protect the ground, especially on plywood or other cribbing to spread the load more evenly, perhaps with logs on top to spread the force out even more. Play around with all the different rigging and log containing techniques on easy trees, so that way you'll know which ones will fit the situation the best and how you can apply them to the harder and bigger ones. Often thinking outside the box a bit can really make your life easier, and using speedlines and pull lines to aim where the piece goes rather than catching it is often faster and easier and safer in the long run.