Skarner loves attack speed. He scales with it better than he does AD. I use AS marks, Armor seals, Scaling MR glyphs, and movement speed quints.
I also use 21/6/3 in the masteries. A little unconventional, but I like the bonus movement speed from the Utility tree. In Offense, take Blade Weaving and Spell Weaving, as Skarner is just an ability spamming, auto attacking monster. The only monster killing mastery I take is Butcher - Skarner has an extremely fast clear time, so you won't need the defensive jungle masteries.
For items, rush Elder Lizard and Trinity Force. If you need early tankiness, build Sunfire Cape after the Elder Lizard, then Trinity Force. Skarner is a fast moving fighter who excels at dueling and picking enemies out, so build around that concept. If you want more sticking power, you can get Iceborn Gauntlet instead of Triforce.
I also typically get Boots of Swiftness, but the other defensive boots can and should be built when needed (Ninja Tabi VS Fed ADC, etc).
In teamfights, you run headlong for a squishy, and ult them. Pull them backwards. If they don't have Flash, they'll probably die. After that, just try to hit as many people with your Q as possible and keep W on cooldown. Use E to catch people trying to run away, you won't need to use it when barring down on someone since your Q comes off CD fast enough to stack your passive quickly.
If you see someone with 3 stacks of Crystal Venom, auto attack them if you can. When dueling, just spam Q and W, and use E when they try to run. Make sure you try and stack your passive to 3 before using your ult when dueling, as it'll proc the bonus damage.
Pre-6 ganks are possible, best if they're pushed up or in the middle (Curl around through the tribush). His E and passive make his ganks good, but his laner needs to bring the damage - Skarners damage is very high when sustained, but his burst isn't that great. Don't do early tower dives, since the build I'm suggesting doesn't give tankiness.
