If you're severely upside-down on a stock that you truly think will comeback, sell it and take the loss and write it off. Wait at least 30 days and buy it back. You can drastically change your basis, while the economy goes sideways. My .02 fwiw.
If you already have losses up to the limit that you can claim on taxes ($3000) and then you sell for the loss, you have gained no benefit. If you then buy it back and have a new basis, when it goes up you will show profits and have to pay taxes on those profits. If you already have your losses, and you truly expect the stock to come back, you hold on to the stock so you do not change your basis. You are stabbing yourself in the back if you sell and repurchase for tax purposes.
Along those same lines, if you own a stock that you truly feel will comeback, and you have no other losses to claim, you could dump off up to $3000 of it in losses, and hold on to the rest, because if it's coming back, you want to keep your basis in it. That's my .02.