Yes, the question is is very difficult.

This is more or less like what I'm thinking up to now:

while(true)
{
    if (hasMessage)
    {
        handleTheMessage();   
    }
}

But while I'm receiving the player's input, I also have objects that need to be updated or, of course, monsters walking (which need to have their locations updated on the game client everytime), among other things.

What should I do? Make a thread to handle things that can't be stopped no matter what? Code an "else" in the infinity loop where I update the other things when I don't have player's input to handle? Or even: should I only update the things that at least one player can see? These are just suggestions... I'm really confused about it. If there's a book that covers these things, I'd like to know.

It's not that important, but I'm using the Lidgren lib, C# and XNA to code both server and client.

Thanks in advance.