1. ssdi is an insurance policy, you have to have a certain number of credits based on wages 5 years prior to the date you claim you were disabled. so you need to check your SSA statement yearly and see if it says you qualify for SSDI based on the most recent 5 years earnings.
If you don't qualify based on current year and still want to file for SSDI, you have to establish your were disabled in a year previous to the current year which can be a problem. SO if your 2012 statement says you don't qualify but your 2011 statement says you do, then you have to prove you were disabled i believe before 2012. not absolutely sure if this is a hard/fast rule but something you need to be aware of.
2. you need a doctor to document your progression. and as you progress and start thinking about disability you might want to have your neuro take notes/do tests with SSDI in mind, a 5 minute exam just to get refill authorization isn't enough. if possible have your GP do a evaluation, can't hurt and they can also write a letter about your disability. if your neuro doesn't do mental/physical testing or charges an arm/leg you might want to find another neuro.
3. keep a log of how you feel, meds.
4. join a support group, there will people tuned into pd that can testify to your progression. if you don't want to do that, don't isolate yourself, make sure friends and/or family see you when your're not doing well since their opinions will be solicited.
5. don't wait too long, if you apply and get rejected you can apply again. i'm a little shaky on this so take it with a grain of salt.

sounds a little strange and depressing but i'm finding that just having been diagnosed with pd 12 years ago isn't going to automatically get you SSDI, you really have to prove your are disabled and it's tough when you live alone and have been self-employed. not encouraging everyone to run out now and get SSDI, just stating that knowing what the application process entails years in advance will make it much easier.

i'm not an SSDI expert, i could be wrong in some facts, so please do your own research, just posting some food for thought.

Thanks ST. Great advice!

I thought the "recent work test" was five years work out of the most recent 10 years preceding the disability (less for younger people). There is also a "duration work test" which is age related starting at 2 years and going up to 10 years for us older folks.

I may be wrong as I've never actually gone through the process.

i may be looking at it too narrowly and you may be right about the 10 years.