I know a tiny bit about this, having helped someone successfully sue and win an appeal.
You know the advice everyone trots out about telling them how it is every day? Yep, do that, do that, and do that again. The reality of living with serious illness is often hidden and it will stay that way unless you spell it out.
The second thing - read the guidelines PIP decision makers use to make awards. Match up the problems you have with the criteria they use.
For instance, if there are some days you can't leave the house from illness, which garners 12 points, tell them that. Go into the nitty-gritty of how you get about, and so on.
The third thing that may help is giving them a bit of background about you and your work to show you're not a waster. Yes, I know - this is a sick joke to anyone with health problems but I reckon it helps. I made sure PIP knew full well I was helping a professional with 20 years as a taxpayer under his belt before making a claim.
We said on the form, look, if you think swapping years of a successful career for living it up on benefits is a choice then you're not friends with the right people. But the main thing is - read the list of what they award points for, then match your symptoms to the points.
If it doesn't work, you can appeal - do it. Over half appeals win, and it's quicker than the initial process.