Valium or, if you aren't keen on the sedating properties, propranolol will reduce your physical symptoms.
Other than that, remember that you are in control. You have a 'get me out now' button at all times - and if the feelings start building up, you can tell yourself 'in a bit, I can press the button in a bit'.
Practically, wear easy on and off clothing. Sports wear is perfect for that, as long as it's lycra and has no metal on it. You might be able to wear leggings (and socks, it gets quite cold in the rooms, even with the blankets they will have available) with a gown on top. Or even a stretchy t-shirt; if you're wearing them when you get there, it's simple for them to tell you what you can leave on and what needs to be off. A pair of slip on trainers is also good.
I found having my eyes closed made it worse, but opening them once I was in, although there was nothing to see, meant I wasn't imaging all sorts of horrors.
They're loud but rhythmic. You can try counting - annoyingly, they don't follow a steady pulse of beats, but that's distracting in a good way in itself.
I've had quite a few. Some have been far easier than others - for me, the hardest ones were a narrow bore one in a darkened room (came out of that one like a rat out a drainpipe) and, weirdly, the one where I only went in as far as my lower legs - although that could be because I was in a lot of pain in the position they had to support my leg in.
If you're uncomfortable laying flat on your back, it can be easier to have a pillow under your knees and to tuck your pelvis under so your lower back flattens out to take any pressure off your lumbar spine. The staff will help you get comfortable as much as possible before the scan starts.
Use as many calming breathing techniques as you can. Slowly blowing out air through pursed lips (like trying to move a candle flame but not blow it out) is very effective. Also trying to almost 'flop' in resignation so your body is physically relaxed helps your mind accept that you aren't in danger.
And above all, the amount of detail they obtain from MRIs is phenomenal. Tell yourself how great it is that they are able to see in such detail, that it's a good thing you are doing this - that doesn't change the perfectly reasonable anxiety you are feeling, but it's a way of calming yourself. It's OK, this is a good thing, it'll be completed soon, it's alright, I can stop it in a minute, I'm just having a rest whilst they look after me. It's OK. I'm OK.
And then when it's done, have a plan for treating yourself, whatever that entails. A warm drink definitely feels good afterwards, for example.
It'll be OK. 