The trick to claiming is to complete the form as though it is a really bad day.
Sorry, but that is really poor advice. The reason is, if someone claims that way, all it takes is for the DWP to show that there are days where they're not as bad as that and the person has effectively committed fraud.
If you have a fluctuating condition, then fill in the forms with an estimate of best and worse case, for example if it asks how long on average it takes you to go the shops, answer: "15 - 40 mns" (or whatever). This way, the DM (decision maker) knows that this is something which varies.
PIP is so new it's really hard to gauge how hard it is going to be to get it, although we already know that they want to cut at least 20% of the budget down, so it's easy to work out they will do this by dropping as many people as they can off disability benefits. Even as we speak, advocacy groups are still fighting to get the regulations changed for mobility, as the distance criteria was dropped from 50 to 20 meters with no consultation and no warnings when the regulations were laid, which is ridiculous. The reality is that no-one really knows at this point how to best respond to a PIP claim form, so the best advice is to do it in the same way as one would do a DLA form.
Another bone of contention is the Atos (or others like-minded companies) carrying out assessments which are deeply flawed and ignoring the evidence from our own doctors and specialists.
All in all, it's a bleak picture.
However, don't let this deter you, if you don't apply, you can't get.
Remember to list all the ways in which your condition affects you *but you may not even be aware that you are doing this to adapt to it*. For example, I don't know, going down stairs 1 step at a time with both feet because it's more stable that way, or if you have installed grab rails in your bathroom, and so on.
I am a disability / special needs lay advocate, and what I see most often with DLA forms is people having got so used to making adaptations to their lifestyle to manage their issues that they don't even think of it as outside the norm any more, and then they wonder why they get refused by the DWP. It's also embarrassing to divulge all and so we tend to not want to write it all down, but that's a mistake. I say to parents I am helping that if they're not in tears by the time they have finished completing the forms for their kids, they haven't done them properly.
If you need any help, PM me once you have the forms.