Make use of the 12 second period Arcane BarrageArcane Barrage remains active; it can be prudent to save your remaining shots to pick off fleeing targets.
The vision granted by an Arcane BarrageArcane Barrage can help against a juking opponent, or to open a fight on brush you suspect/know has enemies within.
Unless Xerath has built up sufficient cooldown reduction and you anticipate needing the movement speed soon, deactivate Locus of PowerLocus of Power immediately after done casting the first row of active abilities, not waiting for cooldowns. Otherwise it would buy the target some time to run and you cannot even hit them with Xerath's auto attack.
Do not use Locus of PowerLocus of Power for the movement speed boost when attempting to escape because of the channel time to activate and deactivate.
However the speedboost can be used to juke. Enter a brush and then activate Locus of PowerLocus of Power, then juke the opposing champion by running out the way you came.
Xerath has relatively low cooldowns on his main sources of damage, Arcanopulse and Mage ChainsMage Chains, so use them whenever possible in a 2v2, or 2v1 situation. When you expect a teamfight, keep all of your abilities off of cooldown.
Xerath is a good farmer with his long range ability to hit all the minions in a wave at once when Locus of PowerLocus of Power is activated and Arcanopulse is used while locked. After injuring the wave and waiting for a few seconds he can clear the whole wave with Arcanopulse once again.
Choosing which boots to buy depends on what Xerath needs. Ionian Boots of Lucidity is great if you like to use your spells more often or if you miss your spells a fair bit. Sorcerer's Shoes are great if you land your spells frequently and want to do more burst.