Had my Class 1 Initial yesterday. I opted for smart casual, i.e. no tie but no jeans. The other guys there (not for initials - one was a controller and I think the others were down for specific bits of renewals) were in jeans, tracksuits etc.
They're all very helpful and you've nothing to worry about. I don't think there's too much you can do to prepare beforehand - just turn up, relax and try to enjoy it.
I've had LASIK eye surgery so the bit I was not looking forward to was the eye test. This is very in depth - I was in there for over an hour. Mots of it I've seen before, but there's a test they do to check your contrast and peripheral vision which involves you staring at a computer monitor and watching for flashing 'C' characters that I found very tough.
The EEG was strange, but as a doc you'll know all about this.
They won't actually tell you on the day whether you've passed or failed as the EEG and chest X-ray have to be interpreted, but I was receiving positive vibes.
Got to say that I rather enjoyed my day out of the office, mainly because it involved taking the shuttle from Manchester to Gatwick. Coming back I had a chat with a My Travel captain who was on the same flight. Like most I meet, he didn't have a bad word to say about his job. Reminded me what it's all about.