Mages like Xerath, Lux, Syndra(maybe not as much) but basically immobile mages, don't roam much. You only roam as a mage if you poked the enemy out of lane, to do this, especially against assassins, simply AA them and hit them with your easiest to land skill shot continually, but only when they try to CS, this way they miss CS, can't push the lane back, and risk dying, all at the same time. So for Xerath, zed walks up to his minion wave to farm, press W the big circle hits him, if it's possible to land your E, do it, then your Q. Then he should be about 75%, if you can't hit E, then simply repeat step 1. If you did do it then now he has only two options, Back and miss CS then roam(punish him hard if he does this.) or stay and try and get the CS. In both cases, he loses something from it, so if he chooses option 1, first push his wave under tower, if your high enough health, roam before he does so he can't do anything at all, if it isn't possible for you to roam, simply back and ping. If he chooses option 2, roam before he does, this way he's way to low to go and roam with you, and if he backs or roams he loses way to much.
Basically it's all about how you play your lane, when i play assassins myself i always hate a good Syndra, Lux or Xerath, because i basically can't do a damned thing if they play the lane right.
Last tip, ward in places where you can see a lot of things all at once, mostly a control ward, this way you don't have to be as scared as usual and make sure to ping your allies to do the same, and most of all, the second you see your enemy laner back, ping it this way they know he's backing and can hopefully be ready for a gank bot lane.