Hello Fla and Ex4Gold,
You might also consider one other alternative if you can afford it. Let's assume you own 100,000 shares of a stock at a higher price, but wish to establish a tax loss. You may not wish to sell in fear of seeing your stock move up without you invested in it.
My suggestion would be to invest into another 100,000 shares, wait the 31 one days for IRS allowable holding purposes, and sell off your original 100,000 share position. You are allowed the tax loss on your original investment, and you of course still own a 100,000 share position. You have established your tax loss without the chance of your losing a potential movement upwards in share appreciation. AND if your shares move up during those 31 days, you may wish to get greedy and end up keeping a winner which used to be a loser!
But as always, check with your CPA or tax attorney (in case I am wrong)
Take Care