I bought Xerath at level 5 and since then I've played him almost all the time until I find something I'm better at. However, in the meantime I've learned a whole lot of tricks as Xerath, including shooting enemies through walls, killing enemies behind turrets, and other stuff like that.
For this character I use ability power glyphs and seals, and magic penetration marks and either ap or mp quintesences.try to balance the two, but mp is slightly more important for this build, since it's meant to punch through the enemy defenses like a sniper or a siege tank.
Pretty much use the 9-0-21. I'd say master clarity, since Xerath is a hog for mana. As long as you get the mastery for that valuable magic pen.
The items are a pretty odd combination. After the first magic pen on the sorcerer's shoes. If the enemy team is building magic resist get void staff before hextech revolver. We don't go for gunblade though, and will is more of a later thing. A lot of these items are from Xerath's recommended list. Blasting Wand is in a lot of these items.
Focus on Arcanopulse and mage chains first. Unless you get within Mage Chains's range, just use arcanopulse/locus to farm/poke on the enemy. Once you have your ultimate, try to use it on the tanks. If you have void staff first, use it quite liberally. If this doesn't kill them, cast mage chains and arcanopulse on them to finish them off. Use Locus if they get out behind their turret. DO NOT use locus to escape unless you are still far away from them.
I'd use clarity and whatever spell you want, but clarity is kind of necessary for Xerath. I usually use ghost, but flash and teleport work good to.