I'm in the (very) early stages of planning the same trip. I start in April, I'm riding down to Morocco first though to do a loop of the Atlas Mountains before heading back to Europe and across to Istanbul.
I've started to keep a little black book of visa information. Anything useful I find on the web regarding visas (embassy addresses, forms needed etc etc), gets written down in this. I'll take the book with me on the road and in theory it should hopefully be of some use to me.
I was also going to post some spare parts prior to departure to someone in Istanbul (using warmshowers/couchsurfing for a contact). From previous tours I've come to find I can usually get 4000 miles out of a chain, a bit less for a rear tyre. I'll need new brake pads by then too and maybe a new cassette for good measure. So I was going to post them to save me carrying what would essentially be dead weight up until then. But as luck would have it I have to fly back home for a wedding on July 31st, so I'm leaving my spare bits at home and taking them back with me.
My route should vaguely resemble this
http://i95.photobuck...ino12/ROUTE.jpg
There's some good info on crazyguy, as already stated.