Be scared of Witch Doctor!

High damage output

Always try to upgrade Maledict to max as fast as you can. It does great deal of damage on low levels and it does even more damage if you and carry keep hitting the cursed target.

Dont bother leveling voodoo cause it does really low healing and burns a lot of mana.

Rush arcane boots as soon as possible cause that item is a life-saver in many occasions.

Keep warding as much as you can, but save gold form glimmer cape.

In teamfights, try to catch as many targets as possible with Paralyzing Cask, cast Maledict on them, cast Death Ward, and cast Glimmer Cape on self. If everything goes right, you can deal great deal of damage, and probably win that team fight.

That makes you a target. You are slow, and you have low HP bar, which means you are an easy target.

If they keep pressure on you, it's not bad to make force staff, or any kind of escape if Glimmer Cape is not sufficient.

The rest of the game for you is trying to farm Aghanim's Scepter. With that item, your Death Ward hits everyone and deals tremendous amount of damage. That's why Black King Bar is also optional, so they can't stop you from channeling even if they use Dust of Appereance while you are in Glimmer Cape.

So basically, if you can farm all of this (don't be afraid to take some kills), your optimal build looks like:
1) Arcane boots
2) Glimmer cape
3) Aghanim's scepter
4) Black king bar
5) Aether lens (optional)