Well, the thing about painting it yourself is that a lot of the different modification a paint job require like sanding or using certain paint \ paint remover might damage the helmet and make it less protective then it's supposed to be.

From what I  know, the only sure way to get a serious long lasting impressive effect is to use airbrush and then bake the helmet in a special oven - both stages take a lot of experience and the equipment is pretty expensive.

We used a small stencil and sprayed a cool bison on Ned's helmet. It came out pretty cool though the paint bleed a little bit, also, it'll be a problem if you want to cover large areas because of the shape of the helmet a large stencil would have to be reshaped and moved and it probably wouldn't look so well. 

Ned's helmet was sprayed a months and a half ago, and still look great.

I think the best option would be to use a decal sticker to decorate the helmet, they came in a million designs, very easy to use and a lot cheaper then a costume made paint job

Look at those!