Next year, I plan to take Oscar with me to Spain.  I have thought about taking him over in the car but it is a 3 day journey.  The flight is 2.5 hours - it isn't going to be a one off, we will do this more and more frequently, hence the sooner I get him used to it the better (he is nearly 2).  I have bought an IATA approved container and in the next 6 months or so I need to get him used to being in it and also get him well prepared for the journey.  I am looking for any tips really on how I can make his first trip as comfortable as possible. 

Hi Scattergood..what breed is Oscar? I have travelled to USA/return  with a mastiff and a rottie,from Italy.(varikennel...in the hold).I have also transported a St Poodle pup from Austria to Italy(sherpa bag,in the cabin).....depending on the size of your dog the place he will have to travel will vary.....I will repost when you have given details of size and weight.
  To enter Spain,he will have to be up to date on his vaccs,have a rabies vacc(at least 6m prior to the date of travel)...a current vet certificate of good health,plus a certificate verifying his rabies vacc is effective(6 m from vaccn)...the latter is assessed by a titre of his rabies anti bodies.Remember in europe there are also parasites that do not thrive in Uk...filaria(heartworm )is one,and tapeworms specific to europe also have to be prevented.
  re the journey
Here are a few pointers...
I would buy the crate or bag he is to travel in ,several weeks before you journey and habituate him to getting in and out so it becomes a safe haven for him.
I found a cool mat(a gel filled mattress) that kept the dog cool ,a literal lifesaver.The heat on the airport terminals can be a killer and you really do not have any means of intervening once your dog is "under cargo",if he is travelling in the hold.
An absorbent liner(eg towelling) will help to absorb any spills or messes.
The water source can easily be tipped over(takeoff etc)..so we wired a square wastepaper bin,half filled ,to the door of the crate ,to ensure dog had water at all times.
A small packet of dog food should be taped onto the top of the crate.This will also deter the loaders from stacking items on top of your dog's crate,.
LABEL;LABEL ;LABEL the crate with your flight details...the dogs details etc.....
I will try to post more once you have given some details of the dog and proposed travel(if it is just for a couple of days,and he is cargo I really recommend kenneling him in UK)...if it is for longer term,do let me know.

He's a miniature schnauzer.  We will be going for at least 3 week's next year and in the future for longer periods.  I would dearly love to have him in the cabin with me but I am sure that the airlines that travel to Spain from the UK won't allow this hence I have bought a good quality transporter for him whilst he is in the hold. I will be taking him to the vet in good time for all of the vacs/tests etc.  The tips you've given so far are really helpful thanks.