SO MUCH good advice on this thread!
Pretty much everything I would have said has already been said, and there's been a lot of consistency in what people have suggested, which is refreshing.
Which marathon (people may be able to give specific advice - I'd say different things about London vs Manchester vs Brighton vs a small race with only a few hundred people in it)?
How long is your plan? I usually follow a 20 week plan (which is one I construct myself based on previous experiences and what I'm hoping to get from the event), but I'd always start it 22 weeks before race day. Then I've got 2 weeks 'in the bag' and if something goes wrong I've got a bit of padding. If it all goes well, then I either repeat a week if I'm feeling keen, or just have a 'rest' week where I keep ticking over and do no more.
The best plans, IMO, are those with a fair number of 'step down' weeks. So your long runs don't go 10/12/15/18/20/22 miles but more like 10/12/15/11/18/14/20/17/22. I find that much easier mentally - getting to the end of a 20 mile run and thinking "I've 'only' got to do 17 next week" is easier that getting to the end of 20 and knowing the next long run is even more. By the time the 22 comes round (after the 17) you've had a chance to mentally prepare (and forget the horror of the 20!!)
Other random things that have got me through (I've done 8 marathons, so have tried various approaches)...
Run your long runs SLOWLY. At least a minute/mile slower than you think you might go. People worry about this one, but it honestly works. (As an eg - for my fastest marathon I did all my training at between 10.30/mile and 11.20/mile and on the day averaged 9.35/mile. I had done shorter runs and some speed work, and it was a pace I was familiar with, but ALL my long runs were done slowly. It means you can run again within a day or so, and your legs are not battered).
Find other people to run/train with if you can - it makes a HUGE difference.
I like to plan interesting routes for my long runs. I figure if I'm going to run a long way, it may as well be somewhere nice/where I don't usually run. Some people like to do laps/use their usual routes, but that is very much NOT for me!
Don't underestimate the power of adrenaline! It gets you a LONG way on the day.
On long runs, and on the day, just take each mile at a time. Or chunk it up in a way that works for you.
I could go on and on, but you've been given so many good tips that I'll probably stop there for now.