Minion waves are like yo-yos. The go, and they come. Once mid-game is triggered you cant stay in your lane farming ,as you said, you need to have a look at those lanes, and guess if the minions are going or the minions are coming. The enemies cannot kill their own minions so if the minions are going to your tower, the enemy cannot avoid it, so you have to wait for the right time, and once the lane is in a good and safe position, you go there and take all the farm and group again or go wherever you should be as fast as you can.
If your support is good enough, he would have warded the way for your roaming to get that farm. If not, well, your cs is good enough I guess, you can survive with that, or you can ward preemptive those roaming paths so you dont depend on your support's work.
EDIT: you need to know when you can do that and when you cant. Use common sense, dont leave your team alone just before a big teamfight because you want to shove a lane that is in a perfect position to be farmed.
EDIT2: long story short. Farming in mid-late is an intermittent process, your way to improve your farm in those phases is to improve your detection of farming opportunities. You do that by knowing wave manipulation so that you can predict the waves position in the next minutes.