What if he/she is different from what I remember?

This all depends under the circumstances which you have first met. If you met in a pub or nightclub and alcohol was involved then the chances are that is a definite possibility for one or both of you!

Let them know what you will be wearing!

You may want to pre-empt this and let the other person know what you will be wearing before you meet up. This will ensure there is no embarrassment for either of you if the other person approaches the wrong person!

Beer goggles

It may be that you had on your beer goggles on the night you met the person however there must have been a reason for you to speak to them in the first place.
They might turn out to be a lot nicer than you remember and you will be glad that you went and met them in the first place. Remember nothing ventured, nothing gained!

What to do if you dont find them attractive?

If you do meet up and find that you do not find them attractive or find that they have an annoying or irritating trait that you just cant stand then it is better to tell them the reason why you will not meet them again rather than leave them thinking something different from yourself. Not everyone is on the same wavelength and people can think disparate thoughts from yourself. It is better to let people down gently rather than build up hopes which are not the same as yours.