Once you're over 40 everything is really subjective, because everyone has weak point and those weak points depend on your particular "mileage/history".
For me, the biggest thing is the commitment to and allocation of time towards my rest/rehab after workouts. The rehab/rest part takes more discipline than the actual gym sessions, because it's 24/7 now. Sleep, water consumption, micro-nutrients (vitamins and minerals), and just knowing things like what part of my body needs a heat pad or what kind of compression sleeves I should use for which joints. Your diet has never been more important in terms of making sure you've got the right stuff to recover. I have a buddy who is 45 and he gets weekly massages from a chick (no, not that kind, get your mind out of the gutter).
I think the second biggest thing for me has been really embracing good form when I lift, I mean precise flawless form. I don't "cheat" any rep anymore (you can't at this age). You need to swallow your pride a bit and go down and get a reputable trainer to watch you lift and critique and correct your form for all the major lifts, might take a few weeks and some money, but it's money well spent IMO. I'll even go back in every so often and hire the trainer to sort of check up on me and make sure I'm still keeping good form so that I haven't fallen into some bad habit again . One thing that has helped me a lot and I credit to DJ Shipley of DEVGRU/GBRS Group fame is the use of the Mark Bell Slingshot to keep my elbows where they need to be on bench press and save my shoulders, I highly recommend the Slingshot for us older guys.
I think some of the best advice I've heard from older guys who lift and have maintained good shape like Stephen Lang is that maintaining flexibility is now paramount. It takes a lot for older men like us to admit that Yoga is good, because it's always been sort of a hippie thing, but let me tell ya Yoga is good for you and get over it and do it. The other thing is that adding reps rather than piling on weight might be a better solution now for you. So, add reps or add sets before you add weight, it's still progression.
Anyways, that's the stuff I adhere to and works for me. As I started off saying at this age everything is very subjective for guys. I find more than ever I'm listening to my body and more in tune with myself ignoring all the vanity and pride #$%* that clouded my fitness vision when I was younger. I take advice now, I take critiques, I don't lift to just be the guy in the gym pushing up the most weight, and I don't view things as strictly linear or that punishment alone equal progress. Everything for me is now about consistent exercise, but the ability to adapt my workouts to recognize my body and what its telling me. I don't just go on a straight routine created by someone else anymore. If I have a body part that isn't up to par on a particular day due to some soreness or whatever, I listen to it and work around it, because it's better not to mess something up and be able to hit it again days later than to mess it up and then take yourself out of the gym completely.