Whenever I go as support Xerath, I buy a gold coin, a ward, and spend whatever's left in health potions. My masteries and runes are the same as my mid Xerath runes and masteries (but I make sure to be extremely silent on this matter, winkwinksuppsetup)
I do my job as support slightly differently to other supports at first: I take teleport (and flash, cus it's the default.) to ensure the lane is never MIA and to let the ADC be more lenient on when to recall and when to farm. I take advantage of this by last hitting everything I can while the ADC is off buying items and slowly walking back to lane. I just poke the low mr enemy ADC until they recall or die to put them FAR behind. My mid setup makes my pokes ridiculous at early levels, and the fact the ADC is almost guaranteed to be AD and not MR (Hello Galio and Kassadin ADC, wait, you don't exist.)
Leaning phase ends with me having 3k gold unused because I poked the enemy so hard they never damaged me more than my health regen and occasional potion could take, stopping me from needing to recall when teleport was on CD.
If my ADC is a hyper carry who doesn't look quite strong enough to survive on her own mid game (Damnit, Vayne, I don't want to be your baby sitter), I buy twin shadows, an item that benefits both of us. I repeat the teleport-farming scheme and then I continue to the next step of my diabolical plan.
If the ADC is alright, l get on with it and continue to the next step.
I buy Luden's Echo. The gold coin mixed with only 30-60 CS is enough for this bad boy. From there, I just use one Q to wave clear and get all the kill gold. If I haven't bought twin shadows, I buy it anyways to help my skill shots, chasing, and for the ADC.
From there, I build as if I was mid the whole time, pushing all the lanes to their dooms, with everyone aiming for the over leveled mid or top while I secretly buy rabadon and void and become the third carry to destroy late game. The only difference between mid and I is the fact mid gets an extra level and an extra item's worth of gold. I get an ADC hugging me while we set the enemy on fire and take over the world. I also ward, but I always ward as mid, so I don't count it as a difference.
Of course, I make sure to be more threatening than the ADC by hitting everyone in the center of my eye of destruction of doom W, so if someone falls behind, it's the Guy Who Was Meant To Support, not the actual carry.
I think my win rate as support is actually higher than my mid win rate, so I'm pretty happy with my strategy.