Adding storm windows
Reduce air leakage and some heat transfer
Caulking and weather stripping
Reduce air leakage around windows
Using window treatments or coverings
Reduce heat loss and/or gain
However, if your home has very old and/or inefficient windows, it might be more cost effective to replace them than to make these improvements. New, energy-efficient windows eventually pay for themselves through lower heating and cooling costs, and sometimes even lighting costs.