1: Animate in 20-24 FPS
2: If you use tweening, make sure to use the “dont tween this frame” option every so often when you need to (its in the frame tools!). Do it when making a punch, on the last frame before the fist makes impact make sure to click that “dont tween” button!
3:Make sure you ease your motions. Not just regular easing, but some occasions, like when making something bounce, use a bounce out. Maybe an elastic out for a car slowing down. Also, when doing these, use regular physics in it like momentum, friction, and inertia.
4: To make movements slower, theres a button called add tweened frame. This does not make your animation smoother, but rather adds an inbetween frame into the timeline that leads into the frame infront of it. The only use is not making movements slower, but helps with easing alot. Be careful and dont overuse it though, some things wont work well with it.
5: Walk cycles are easier than you think. On ralphs youtube channel (named FTLRalph.), his latest video showcases not only how to use movieclips well, but also a walk cycle tutorial which is pretty easy to follow. Instead of making a tweened animation with a low FPS though, you can just add alot of tweened frames to make it slower, like how I said earlier.
6: Ask for advice! This applies to story animations, with lore. Make sure to iron out plotholes in your animations stories as much as you can. Trust me, you will need to.