Individually there is not a great amount you can do unless you want to put some serious effort in (organsing events etc), but there are a few simple things. If you go to
www.bfpo.org.uk there's a bulletin board affair where you can send short messages to the lads (not just in the gulf), which is becoming quite popular so long loading times are to be expected. You can also send Emails to various news places supporting the troops - it might not do very much but we can at least give them an excuse to say "we've had oodles of messages of support for the troops" which might help a little bit.
Ideally some well known and charismatic people could organsie a rally in support to counter all the anti war protests, as I've hardly seen any marches in support of anything lately. Or perhaps get together with some friends and run a charity event for the Royal British Legion or somesuch, it might not directly help the men but the it's the thought that counts. Generally anything which can get noticed by them to show we care, as a number of times unfortunately I've seen interviews from the gulf of blokes saying they don't think public support in the UK is very great, and it would probably be a morale booster to some extent if the country as a whole could show that's not the case.
EDIT: Please be aware that a highly intelligent virus called Rodney had, for the time in which I was typing the first half of this post, infected my hands causing them to go all wobbly; I am happy to report however that I am now fully recovered and have corrected the obscene typing errors but if anyone has any lasting psychological scars and needs therapy to overcome the sudden influx of meaningless jibberish, the right honourable Mr. Parry will gladly assist with the aid of a sharp scalpel and a pot of engine oil.