proper session handling

I am working on a membership system, and I'm not quite sure how i should manage my sessions.

Right now i am getting errors when more that one user tries to log on at the same time.

How can i keep my $_SESSIONS flowing smoothly?
When should I unregister a session?

Having some trouble managing errors, messages to the user, and and multipul users...

Can anyone provide a few coding seggestions?

I'd post some code but there is way too much. Thank-you!

[edit] I have been told lately that this is bad! and should not use this...

        // That it is out dated...  

        session_register('userid'); 
        $_SESSION['userid'] = $row['userid'];

.. And that you should only use $_SESSION, so what are some comments with that as well.

My Main problem throughout my site is consistancy! i switch back and forth depending on what tutorial i used to help me write my code...

Please make soem seggestions! thanks again!
[/edit]

We can only help if you show us the code! As well, what errors are you getting...copy them verbatim

well how would you all seggest mapping out and managing the sessions throughout the membership system.

I'm just looking for some advice, tips, tricks, and smart prevenative coding techniques.

I'm just not confident in my solution. if you really want i'll post it.. but it's ugly and not a finished product..