Be prepared - know in advance how many numbers/cards/whatever you're going to try to memorise, whether you're going to just look at the numbers once, or revise them one or more times, and so on.
Keep an eye on the time - verbal reminder will be given (eg "five minutes remaining"), but most competitors like to bring a stopwatch so they can see exactly how the time is ticking away.
Know how the scores work - remember that in most of the events you need to memorise the information in sequence, starting from the beginning. It's better to do one row of numbers, column of words, pack of cards etc and get it completely correct, than to attempt ten rows and have a couple of mistakes in each of them.
If you've practiced with your own playing cards and recall papers, it can be a help to bring them to the competition to use instead of the ones provided. A slight difference from what you're used to can be surprisingly jarring in a competition.
Relax! This is a very friendly kind of tournament, and you'll get better results if you just take it easy and see how it goes.