This. I'd add that your players also could make your life easier if they told you in advance which plot hooks they intended to follow. That lets you focus your limited prep time on what the players actually want to do for the next session or two. You can put off developing the rest until such a time as the players get around to pursuing them.
Yes, yes indeed. I tend to ask my players what they want to do next as early as when they're packing up after a session, and harass them again so I have a general idea of what their next plan well before I have to start planning the next evening's session. If they request more information to make an informed choice, I try to give them a paragraph or two over email, hopefully seeded with enough intriguing emphasis that yes, this is an interesting plot hook, that they bite fully.
As for specific tricks to mid-level plots apart from "carefully tail player motivations..." Mostly I tend to start exploring possible connections between smaller plots, or hooks the players pick up on. For instance, I knew for one game that recently I wanted to have a troublesome bandit problem, and I wanted to have an agriculture-goddess temple taken over by plague-priests. The "mid-plot" came about when I started trying to figure out how to link the two. Before long, I put together something about a land-grab in which the bandits were paid to harass this one estate, and the "backup plan" was to hire the plague-priests to blight the land (temporarily, of course). Of course, plague magic is a little more dramatic than a blatant land grab. So the person who wanted the land was clearly more of a patsy in the grand scheme of things, and his realization of that might lead to some interesting desperate moves on his part, and stir up more activity from the pestilence-cult... and after a bit of musing, bam, there I had something to tie together several weeks of adventure.
They don't just have to be adventures, mind. I tend to keep a small book in which I jot down any idea that occurs to me as befitting a given theme. So, for instance, I might jot down "succubus imprisoned in a mirror," and then later decide I want to tie that in somehow. The structured challenge of using a "secret ingredient," Iron DM-style, will sometimes provide interesting ideas for ties between otherwise unrelated things. Of course, it helps to have a long list of ingredients so you can find something that works. But that's why I keep the book around at all times, so I can grow those lists at any point.