WARM UP thoroughly. Get your blood moving, do some mobility work, stretch, foam roll, use the lacrosse ball, everything. I started doing total body warmups for every session, even if it's only a bench day. I have had a lot less joint pain and muscle "tweaks" since.
Cycle your tricep work. If I do heavy tricep work for too many weeks in a row my elbows will flare up something fierce. Now I do heavy extensions/lockouts/dips for 3 weeks and then back off with cable/band pushdowns. This has kept my elbows fresh.
Always respect the weight. I've seen guys get injured because they got sloppy on a warmup set. I've had this happen to me, when I tweaked my pec on my last bench warmup set because I rushed my warmup and my concentration wasn't there, so my technique was off. Always treat every set with respect, no matter the weight. Even if you're only using the bar, do every rep perfectly for practice.
Second Zep with the machine work note. Machines can be very useful for certain things. I like the Smith for Military presses, CG bench work, and inclines. I love Hammer Strength back machines. And the leg press has its uses for training the legs while deloading the spine.
Working your calves can help your squat. I found this out recently. Building my calves up has helped my stability and with walking weights out of the racks. I don't really know why, but it has. I think this would not apply if you were a super-wide stance, mono-lift only squatter though.