I had a DLA medical about 2 months ago - and have been awarded something (I have recieved some back pay but no paperwork yet!)
A doctor visited me at home - they don't always do home visits, but if what you say on the application is bad enough, they realise you can't trek halfway across the county to their offices.
The Dr asked the same kind of questions that are on the application form - how far can you walk without severe discomfort/resting, can you cook a meal etc. (if you have a copy of your application form re-read it to remind yourself what questions there are).
then she briefly examined me, looking at things like muscle wasteage and movement problems and dislocations etc. If you get asked to do something which you know is going to really hurt for the next few days - tell them - don't just do it. I refused to touch my toes because I think the extra pain would have made me throw up, and the doc accepted it.
I don't think the doctor sees your application form before the visit, I think they just get the diagnosis (somebody shout if I am wrong..), so I expect the assessor will also look at whether what you say matches. Don't miss things out because you feel it is whinging - you are just stating the facts, and yes, sometimes facts suck!
If there is someone who has to help you with things in normal life then it is handy to have them around. they can help answer 'what does she need help with' questions. Also to give some moral support.
I was having a genuinely hideous day when the doc came, and I think it helped because it was pretty clear there was something quite wrong with me. don't try to put a brave face on (which I sometimes think is quite difficult once you're in the habbit
The doctor asked me at one point whether I would recover. I said I had no idea, maybe I would improve slightly, maybe I would improve a lot, and maybe I would get worse. It is impossible to tell with this HMS. (yes I intend, through hard work and excersice and stuff to improve, but a DLA application is not the place for optimism - keep that for real life!.
Ok, so now I have really written a load of nonsense. basically, it is like an interview form of the application, be yourself, and be brutally honest with how life is on bad days - they will only know if you tell them, and a friend on hand is always a plus!
Good luck, I hope it goes really well for you cos you are due some good news.