Examples of other movements...
Lower body assistance: RDL, Leg Press
Lower body minor assistance: Leg extensions and/or curls
Upper body assistance: Handstand pushups, pullups
Minor upper body assistance: Lateral raises, bodyweight rows
Every year I spend lifting weights adds more tools to my toolbox. Most of these tools either don't work or only work for very specific purposes (see: peaking strength and size), but it's easy to overlook the tools that truly worked well for me. Here is a short list of what has worked over 2016 and why.
This feels insane to write down. I used to be all about stretching. As in, I would do splits while working on my math homework into it (how the fuck).
For a few years I felt as if I needed to stop doing heavy deadlifts (and sub in oly lifts) in order to feel 'springy' or athletic. The truth is, my hips were just tight as hell all the time, and I did nothing to help them until an injury forced me to.
Stretching 10min a day has been a godsend. All of the usual nagging pains in my hips and knees are absent. I feel as springy as ever without having to leave deadlifting behind. Consider static stretching extra assistance work at the end of each training session. If you find it more boring than a high school reunion, it helps to do sets of arms (see: hammer curls, lateral raises, some kickback variation) in between stretches.
I've seen an immediate and consistent improvement in my quality of life and quality of lifts for minimal effort.
2. Specificity Rules
tl:dr Do a few things really well to expand your base of strength.
This is linked right up to body dysmorphia. The feeling that 'if I don't do X (which helped me get this big) I'll lose everything I worked for.' In trying to be good at everything, you'll end up being mediocre all around (except on some lifts which will thrive no matter what). Ask yourself, do I really need to be strong on X right now? Am I just afraid of not being strong on X anymore?
This feeling is not entirely wrong. If I do floor press and dips on my upper body days, I'm going to progressively get better at both at the same time. If I remove dips from the equation, my performance in dips is going to drop dramatically. Well, so fucking what? Concentrating leveling up a few movements is going to ensure that your nervous system gets a hard on for that movement pattern. I'm going to make my shoulders, chest, and triceps strong as fuck on that single movement. When I eventually come back to dips, the movement pattern will be weak but the muscles will be strong. Within a week I'll be where I want to be.
Your assistance work can fill in the gaps to work your muscles, tendons, and ligaments outside of the movement pattern you're trying to improve. Which brings me to...
3. Assistance work. It's kind of sexy yo
tl:dr Powerbuilding via intensity then volume is good hurghhhh.
Just because you're specifically trying to get better at a few movements by no means states that you can't venture out to other exercises. After my main intensity and backoff work, I like to add a shitton of volume with exercises that force me to feel my muscles working. Whether it's 100 rep sets of leg extensions, shoulder machine presses, or bodyweight rows, the idea is to get a pump using the same bodyparts I just thrashed to hell.