Ive been playing for some time and decided to share some of what I have been doing with Xerath.
Xerath is fairly tanky with all the free armor so I build on that as a secondary focus. His main focus should always be AP and damage.
2/10/2012 - reduced it down to one build and started running swiftness mastery.
8/2/2012 - minor tweaks based on recent(-ish) change to flat AP glyphs and other recent changes.
Ignite - Xerath's CDs are too long early game to really stick it to your opponent. Ignite is mandatory.
Flash/Ghost - Xerath needs an escape, especially after rooting himself. The extra movement speed after you turn Locus of Power off is nice, but it is not enough to get you out of a difficult situation. I haven't noticed a big difference between ghosting down a lane through minions or flashing over a wall to escape, especially after the flash nurf, it is mostly a matter of preference. If you decide not to take points in utility definitely go for ghost.
Quints of Potency & Glyphs of Force - These are almost necessary to make the most of Xerath's passive. Glyphs of Force are better because the per level glyphs are just as good as the flat AP glyphs by level 6, after that they are clearly superior.
Seals - Lately I have been using mana regen / level and swiftness mastery.
If you are new to Xerath I can understand using both Meditation and mana regen / level seals for all the skill shots you miss. Otherwise pick one or the other.
Mark of Warding - I know it sounds completely strange that I prefer 7 magic resist to 9 Magic pen. With Xerath's percentage magic pen given to him when you use Locus of Power, Arcane Knowledge, and void staff being a solid option for Xerath, I am not too concerned about magic pen. Early game it is a toss up between the two, deal extra damage or take less damage from the AP mid you are certainly going to face. If you know you are going to lane vs an AD take magic pen, otherwise magic resist is comparable early game and obviously superior late game.
AP should be the main focus of any Xerath player so 21 offense is an obvious choice.
The next step is figuring out if you are going to use flash, Meditate, and which seals you are going to use. You can follow the guide above, but play around with it most of this stuff isnt that mandatory.
Arcanopulse is too strong of a spell, level it first. Locus of Power is a great way to amplify damage from all your spells, gain extra magic pen, and Locus's CD goes down greatly for every point you put into it, level it 2nd. Ult is amazing, just like 99% of the champs, level it when you can.
The only variation I ever use is whether to get Mage Chains or Locus of Power at level 2.
If you are facing a champion that likes to get in your face (vlad, most AD) get mage chains and punish them every time they do.
If you need sustainability Locus of Power is free damage increase, and gives you the range to harass any champ.
Items are the one thing that are not set in stone. Item purchases should always be subject to change during any part of the game. Every time you go back you should always double check your opponents items quickly to help guide you. You are always going to need lots of AP and you shouldn't give up too much of that, even if your opponents are dominating and making you feel like you cant do anything.
Boots vs doran's to start: This is all about how fast your mid or jungle opponent can jump on you and take you by surprise, if this is a threat take boots and some health pots. If you need more sustainability because your mid opponent is just a bruiser get doran's. Lately I have been using swiftness and doran's ring most of the time.
Kage's Lucky Pick - every time I think I am losing or my lane is a tie I get a pick.
Upgrade boots: Ive said it before, I dont think xerath needs flat magic pen, especially by the time you are deciding which boots to upgrade to, so I never purchase Sorcerer's shoes. Ionian Boots of Lucidity are my staple boots and I purchase them the majority of the time. The other strong option that I get is Mercury Treads. I am not a fan of Moonflair Spell blade for Tenacity, I would rather find a different CDR item.
Ryali's Crystal Scepter - Too solid of an item to pass up, the slow will help you land the stun. The AP/HP fits the theme of tanky DPS.
The next item I usually get is either Rabadon's or Banshee's depending on if I am facing a ton of AP or AD. If I am facing both I usually get Rabadon's first then Banshee.
If you are wrecking them they are probably purchasing magic resist. If you notice this, Void Staff is the only way to go for more percentage based magic penetration.
If you are having trouble getting or keeping blue buff consider getting a Deathfire Grasp, I like the click effect too much to get the other similar item, Morello's.
If your opponents have tons of HPs (at-mog builds, cho, ect) Deathfire Grasp's percentage damage will do wonders.
I almost never buy items with armor, even zhonya's hourglass, I usually get around 100 armor just relying on xerath's passive.
Plz rate if you enjoyed or hated this guide!