1) Pay the Jan mortgage by Dec 31;
2) If you own small business, buy the stuff you need to buy anyway by Dec 31;
3) Sell the losers in your stock portfolio: I mean those true losers that you don’t expect them to come back. Some people sell their losers and buy them back in Jan. Perdsonally I never tried that, and don’t recommend it (there is possibility violate IRS free ride rule). Another strategy, which avoids messing around with free ride rule, is sell a stock (say GE), and buy a similar stock back (say Emerson). This may work better for mutual funds (e.g., there are many S&P 500 index funds).