Most new hammers have a clear coat on them, take a wire wheel and buff down the head of the hammer, once the clear is removed it will weather... Just Google up some weather techniques for steel, from that point on... Acids, salt water, and even just dropping it in some mud outside for a few will get natural weathering...
For the handle I would first strip the clear coat, just regular furniture stripper will do the job, it might even help weather the head... Anyway once the clear coat is removed, I would likely take some misc object like a big hunk of heavy chain, and whip the wood to distress it and add character, and then a little sanding to knock down any splinters... Maybe run a flame over some areas to add color... I would then proceed by rubbing a combination of wax and oil finishes, again consult Google for oil and wax finishes on wood or furniture to get some ideas...
And last but not least once the head and handle are knocked down and aged, I would get some sinew or other 'old style' lacing from a leather shop or online... Take a trip to a place that sells dog treats, and get a bigger rawhide bone, maybe get a few different brands and sizes if they have varied thickness so you can experiment...
Once you have the lacing and rawhide, you can create a wrap of some sort on the wood handle... Some things to note, rawhide is organic, so this will 'stink' a little until it dries back out... Anyway soak the rawhide bone in hot water, you might find it best to simply let it soak overnight, whatever it takes to get the rawhide rubbery and flexible again... At this point you can tea stain, coffee stain, walnut chip stain or whatever to give the rawhide some character and color... If you wrap the handle with the wet rawhide and lace it up tight while wet, when it dries out it will shrink into a form fitting hard shell around the wood... And don't leave the wet rawhide soaking or laying around too long unless you like NASTY smells
you need to get it dried back out or it really starts to stink in short...
Alternately you could use leather instead of rawhide but the rawhide IMO gives a more primitive look...
Here are some examples of rawhide wraps, you could do it like this or wrap a sheet with holes and lace it up like a corset...
http://i37.photobucket.com/albums/e77/wicklrb/0a754675.jpg
http://i747.photobucket.com/albums/xx118/churchandson/Trade knives/yswh1.jpg
http://i747.photobucket.com/albums/xx118/churchandson/Trade knives/yswh7.jpg
You can also lace or wrap some rawhide strips around the head of the hammer like this...
http://i747.photobucket.com/albums/xx118/churchandson/Trade knives/yswh6.jpg
