Dynamic file directory for file fields
It's very easy if you have the Token module installed. On the file field edit page you'll see something like this:



The big table is a list of tokens that you can use in the path for the file. By default there are tokens available for current user, current page, current date and a couple of others.

For example you could have a path such as users/[current-user:name], in which the token would be substituted for the current user's name. You just need to click in the text box you want to add the token to and then click the links in the table, it will auto-fill for you.

Hope that helps