I have a blonde wig for my Seras Victoria cosplay.
On the back of the wig, it flips out. (Like Honey Senpai)
I wanted to use the wig for Kurapika first, and flatten it.
I would probly buy another wig for my Seras cosplay anyway.
The reason I dont just buy a new wig for Kurapika; is that the wig is poorly made, and when I put it on, you can see my black wig cap in the back because it flips up, and seperates. It only looks better, and covers more if I flatten it with my hand, and look in the mirror.
My boyfriend bought it for me, so I want to make use of it.
Would love to cosplay Kurapika sometime, so I want to know how I can flatten the darn thing! xD

Image of Kurapika http://cosplayisland.com/costume/view/52229

Wash it.

Here's a bit more detail than "wash it".

The tutorial actually shows how to detangle a wig, but it'll also straighten one :3

EDIT:

There is also this which is just warm water

Another with fabric softener

Depending on what fibre the wig is made of you should be able to style it flat with heat, but do be careful incase your wig isn't heat resistant. Test a little piece where it won't show if in doubt and work a little bit at a time.

-Hairdryer on a low temp setting; if that alone isn't enough you could wrap it round a brush or a large curler at the same time to shape it against.

-Straightners on a low temp.

-Hang under warm running water or wash and hang.

Low temp is the important part, there needs to be heat but it can't be anywhere near as hot as you would use on real hair.