This tutorial assumes you already know how to vector mask/cut using the pen tool
the original:

so you're vector masking steve, but the hair is a nightmare.. even if you were able to plot the million and one points to mask it accurately you'd still get mixed results with vector plotting at the 1 pixel width level. There is no way to vector mask hair this messy.
1. So we pen tool around all hair.

so clearly we need another form of masking.. if you weren't already aware each layer can have both an alpha mask and a vector mask (and if you ever need more you can just have nested masking by applying additional masks to layer groups.. and I'm not even going to mention clipping masks). If you don't know what alpha masks are then simply put they're like a reversable eraser using (primarily) the paintbrush tool to designate transparent regions by painting black (for totally transparent), white (for totally opaque), or 254 shades of gray (for everything in between).
To create an alpha mask for the active layer either click the "add a mask" button (the grey square with the white circle) in the layer tab or from the menu select layer > layer mask > reveal all (hide all does the equivelent of holding shift and clicking the "add a mask" button).
It would be nice if perfectly masked hair could be produced by just applying two layers

but it doesn't really work out as the image above (jessica alba from sin city in case you were wondering) illustrates.. basically it's an up hill battle to isolate hairs. you spend more time trying to get the masking right than actually masking and even still you can't entirely eliminate the background. clearly a work around is needed.
the solution is to make the background more or less exactly like the foreground and then mask that.
2. Duplicate your steve nash layer (you will need the original as a reference later).
3. In the new layer pull out the paintbrush and start filling in the areas around the hair with same colour as the hairs adjacent (hold alt and select regions to get the colours with the eyedropper).

it doesn't have to be perfect because all we're doing is eliminating contrasting backgrounds. We're still going to mask the same hairs so what we're painting will largely go unseen (it just wont look as obvious as the old background). You can be as accurate or inaccurate as you like because you probably wont notice the difference in the end.
some bits will be a bit iffy so you might want to use the smudge tool to even out an majorly contrasting areas.. the beauty part is that if it doesn't look right after you've applied the mask you can always go back and tinker.
so you should end up with something like this

so now we have another little problem. we want to mask a layer using a layer that is below as a reference. The problem being that if the top layer starts as opaque then you can't see the reference layer and if it starts as transparent you can't see what you've done.. and I've never really found any of the blending modes to be particularly useful when tracing (which is basically what this is).
The solution:
4. With the layer where you've fixed the background selected hold shift and click the "add a mask" button (or menu > layer > layer mask > hide all) and then add "color overlay" as a layer style (with the default settings).
this way you'll be able to see where you've masked as you go.
5. Now (making sure you have the alpha mask selected (it'll say "<layer name>, Layer Mask/8" at the top of your window) use the paint brush to trace the hairs (which should turn red as you mask them which is just the layer style which we later disable)
I usually start with a larger brush in the center and cover all the areas where there are no gaps between strands and then use a small brush to stroke the individual hairs.. remember this is a simulation of hair. it doesn't have to perfectly represent the original. it'll look fine.. it's all about how much time you want to spend.

it's also important to remember that you can go back at any time and manipulate basically all of these steps, so each step doesn't need to be done perfectly.
once you think it's starting to take shape
6. go back to the original layer and reposition the vertices that are currently outside of the head and place them inside your red masked blob. you can disable the vector mask to view the original background again at any time by holding shift and clicking on the vector mask thumbnail in the layer tab (shift click again to re-enable)

from here on in you can either re-enable the reference and continue tracing or you can leave the reference hidden, disable the layer style and draw your own approximation of the hair (can be a big time saver). make sure when you go back to the fixed background layer (which I called hair) that before you start drawing you click the alpha mask thumbnail to ensure ur editing the alpha mask and not the layer itself.
7. Use combinations of a small paintbrush (1 or 2 px width) and smudge tool (1 or 2 px width with 90 - 100% pressure) to achieve fine hairs. Note: It's a good idea to work with a coloured background as it will help reveal errors which you may otherwise not have seen (create a color fill adjustment layer and throw it at the bottom)
pretty quickly you should wind up with something like this..

for most intents and purposes this is as probably as good as you need for a signature.. from here it's just a matter of light refinements.. I'll be back later with my finished product.
if you have any questions on how to perform any of the tasks outlined in the tutorial or about anything else, please feel free to ask.
if you've done this once or twice then you'll probably be able to do it in under 10 mins..
and the finished product:
