I've had my share of deep divers so I know the problem and now new money goes mostly into ETFs (and not weird ETFs either). AIM after all uses similar principle to dollar cost averaging which applies to indeces not to individual stocks. I think Lichello misleads with his example of the stock that takes a dive and recovers, and of course AIM does well with that, but most stocks that take a deep dive never recover or if they do, do so in a very long time course.
If your deep diver is in a taxable account you can sell a part or all to offset capital gains or income as I'm sure you know. Other than that you have to decide if the stock can stabilize and recover or if it will continue to decline. If the latter, just bite the bullet, accept the loss and learn from mistake.
Adam