Another rule (restriction) you may want to know:
"A capital loss carryover is the net amount of capital losses that aren't deductible for the current tax year but can be carried over into future tax years. Net capital losses (total capital losses minus total capital gains) can only be deducted up to a maximum of $3,000 in a given tax year."
So, you can generate a huge paper loss "for the rest of your life" by actually holding the same portfolio, but you cannot deduct more than $3K per year unless it's covered by capital gains from this or other investments.