I agree you need to preserve assets, but that time was earlier this year. Personally, I thought the big downturn was coming last year, but I was wrong. I lightened up in stocks by 20% the end of last year, and have 35% in stocks and 32% in cash now. I am buying stock funds slowly, getting lower prices. I've taken losses, but that is part of the investing for the long haul.
The best way is invest contributions into the TSP equity funds, or L funds. Be greedy when others are fearful, while prices are low. In the long run the "buy low" approach will always win out. NOW IS THE TIME TO BUY, NOT SELL. You do this by putting new money into the stock funds, not the G fund.