Why is that that I always have nothing to say, or forget what to say, when I'm talking to a person, but once that person has left or I have left them I remember all the things I should have said or wanted to say?

It could be a few reasons,maybe you are a bit nervous speaking to the person facing you or perhaps you have lots of things on your mind at the time so you lack concentration.

Shyness is a huge problem with some people,they plan what they have to say but when it comes to the crunch their mind goes completely blank... we all are guilty of that one.A typical example would be a visit to the doctors surgery,there we sit going over what is wrong with us,the minute we face the doctor and he says how can I help you, most of us blurt something similar out of our mouths resembling our symptoms,we then come out thinking... I didn't tell him this or that, I've done it many times.Many people write things on paper so's not to forget anything that the doctor would need to know,then just hand him the piece of paper,I've done this myself which makes things much easier and much clearer for the doctor.

If you have to speak publicly, then it's a good idea to practice... in the mirror,in front of your family,friends etc just to give a sense of an audience, eventually you will become more confident, relax and therefore remember what you want to say.

Find some common ground with the person you are talking to... wether it be sport, music, books,the latest film on show and have they been to see it or some other interest.Watch the local news reports, find out what's going on in your area.. someone will always have an opinion, which you can challenge if you disagree.As long as you have the knowledge of what ever topic is being discussed then you will be fine.

You could always use a crutch to help you, e.g.. put some information onto your mobile phone, which you can have to hand... imagine your standing trying to talk to someone, when your mind goes blank,realizing this you quickly say... can you just give me a minute,you glance at your info which triggers your memory again... you say, sorry about that thought it was important or what ever comes into your head,then carry on the conversation.The pause will give you time to relax and collect your thoughts again.. comedians use a similar thing,just to buy them a bit of time.

Hope this helps a little.