Forgetting your lines is every actor's nightmare. In the business, this is called "going up" on your lines... and it happens to everybody at some point or another. Of course you should always be prepared, know your monologue inside and out, be able to recite it in your sleep, backwards, on cue, etc. But if you do go up in an audition, do not panic. Act professionally. If you are doing a piece no one has heard of, improvise a few lines until you get back on track. If your piece is well-known, then pause a moment to gather yourself, but stay in character. This is vital. The auditors are looking at you as an actor. Your composure and confidence can sell you on the rare occasion your memory lets you down.