I went from 50 to 100 and here are a few things I found out.
Not accepting a seat at checkpoints and staying on your feet I think helped, you'll spend enough time sitting down in the weeks after the race anyway.
Sort out foot problems immediately no matter how small.
visualise yourself crossing the finish line beforehand and flowing through checkpoints efficiently. Its easy to waste loads of time here, I picked up food and ate on the go at all but the halfway one, here I spent 15 mins eating a hot meal (standing up).
Think positively, each step is taking you closer to the finish etc.
Encouraging text messages in the middle of the night made a big difference to me, I was alone for 60 miles apart from checkpoint staff so it was nice to have some contact!
You can't guarantee where your highs and lows will happen, my low was at dawn and lasted a few hours. The only guarantee is that things will get better (although you may have to go a bit lower first! sorry).
I will see some of the 100 milers as they pass my house at Presthope on Wenlock Edge, not sure if I will see the 50ers but I may try to get over.
I read that ultras are 40 per cent physical and 60 per cent mental and I would agree with this so as long as you have done the training you have just got to sort your head out!