Mostly wanted to say that there are some great, well written, thought out and supported posts in this thread. For better or worse, that's not always the case on forums. Back in November, I just started lifting again after four years away from a gym. I've lifted off and on for over 20 years (42 now), and while I've always gotten results, I also think that there are always new things to learn. So thanks to all above for your insight and information.
My new thing that I've never done before is to incorporate
stability balls/bosu balls into my weight lifting. For example, instead of doing flys or dumbell presses flat on a bench I'll put my back on a stability ball, feet on the floor with knees bent at 90 degrees and bridge my body. Then, doing the motion with the weights you can feel your whole core trying to keep you in place instead of rolling off of the ball. When I first started these I had to reduce the weights by 25%-50% over what I would do on a fixed platform. On some days, my core ends up feeling more sore (in the good sort of way as opposed to pain) than if a do a targeted abs day. And I've never loved doing push ups as much as I do now incorporating the stability balls into about 12 different exercise forms. There's plenty of information as to exercises and form to be found on a search of the web.