Take any opportunity to hit your opponent with Power TransferPower Transfer, although bear in mind its mediocre range may put you in a dangerous position.
Keep in mind that offensive use of Gravity FieldGravity Field in a lane is very limited, as it costs a lot of mana and has a too short range to be used safely in this manner. Try to use it for zone denial and as a defensive measure versus ganks, rather than relying on it to set up combos.
Death RayDeath Ray can be used to quickly clear waves when returning to base, or preparing to roam and gank another lane.
Death RayDeath Ray is also a useful long-range poke/harassment tool, although its manacost is fairly high and should thus be used sparingly.
Viktor's lane presence increases significantly when he gets his ultimate Chaos StormChaos Storm; combined with his other nukes and an Ignite he can easily burst down most other mages.
Points into Death RayDeath Ray will also increase Viktor's DPS with higher damage and reduced cooldowns. However the main differences are the casting range and AoE; from max range Death RayDeath Ray can allow Viktor to damage multiple enemies without any chance of retaliation. A well aimed cast (with some ability power boosts) will also clear creep waves with ease. However, the mana cost increase and skillshot requirement are notable downsides to maxing Death RayDeath Ray.
In higher level play, you will want to max Death RayDeath Ray for the majority of your games as it is Viktor's wave-clearing ability, allowing you to quickly farm a wave then move out to take map objectives or gank.
Your choice of upgrade for The Hex Core is a major factor as well, since ranks in the Augmented ability will increase the item's effectiveness drastically.
Smart-casting Death RayDeath Ray is very different from most other abilities, and will take practice to perfect. One method is to place your mouse at the starting point, then "flick" your cursor in the direction you wish the laser to travel.
When escaping remember that the shield from Power TransferPower Transfer is based on the damage it does before reductions, so it would make no difference in shield value if you aim at a minion rather than a pursing champion.
Rylai's Crystal Scepter grants a substantial amount of health alongside AP. More importantly, its slow effects make your Chaos StormChaos Storm even more potent as enemies will take more damage trying to escape. If you are close-up to the foe, Chaos StormChaos Storm's increased movement speed and chasing ability can force them to take all of its damage.