In reply to greg_may_:
I'd agree with most of that. Especially getting nav right - on our first MM, we walked throughout but still got into the chasing start on the 2nd day due to accurate navigation on Saturday (and then dropped several places on Sunday due to inaccurate navigation!)
For a first event, if you're not sure on whether to take something - then take it. It's far better to learn what you can do without by carrying something and not using it, than by not carrying it and finding you need it. You can then start ditching things before your next event.
Bags for your feet at mid camp. I always used to do this (the LAMM even give you special bags for the purpose). But I realised that my socks were getting wet anyway due to sweat+condensation. So these days, I just wear my fell shoes without any socks when walking round the camp, and save my dry socks for when I'm in the tent. For summer events I don't even both with spare socks, but for the OMM I do.
Regarding course choice. Quite a few people (including some of the OMM team) recommend choosing short score for a first event. This has the advantage that you'll know how long you'll be out for, and can always miss out a control if you can't find it. On the other hand, score courses require an extra ability, namely to know how far/fast you're likely to go, and also need much more careful map reading to avoid an apparently short easy leg which actually involves some big reascents.
The other consideration with course choice is the number of other competitors. For C, you'll get long lines of people between controls. This has the advantage that you can just follow and not worry too much about navigation. And the disadvantage that you're relying on the people at the front of the line to know where they're going! And of course if the clag comes down you'll lose sight of the others (though you'll still often get trods to follow).
For B there are still crocodiles, but with A you're likely to spend most of the weekend on your own (and anyone you do meet will probably turn out to be on a different course).
Finally, if you haven't already, take a look at previous years' courses at
http://www.omm.routegadget.co.uk/