What is the best way to submit a story for review?
This is up to you and your own preferences; however, I suggest you either attach it to a thread as a simple .doc document or paste it in a thread. Reviewers don't always have the time to return emails or the desire to give our their address, so making the work more accessible encourages responses.

May I submit a story each time I create a new draft?
Of course; the editing process is about incremental change, not a single frenzy of adjustment. I do, however, request that you keep each work to its own thread for consistency (otherwise it may become hard to tell which draft is the most recent) and also to avoid cluttering out other authors' threads. I also encourage you to attach a link to your finished story once it's been posted.