If you get a big income tax refund every year you may want to consider adjusting your federal withholding (W4) and rerouting that money into your TSP. I increased my contibutions by 2% with $3 decrease in take home pay. No more lending uncle sugar part of my paycheck. It takes considerable calculations to avoid getting yourself into a position where you'll owe taxes, especially if you have several sources of income. The IRS has withholding calcuator on their website.