Warning: This might make you cry.
Since we've had such a rip-snortin rally as of late, lets figure out where you stand, minus contributions, since the closing high of 1565 on 10/9/07.
The S&P, from 10/9/07 to 1/11/10 (latest high of 1147) is down a whopping 26.7%.
First, log into TSP, go to account balance. On the bottom you can enter a different date. Record your balances on 10/9/07 and 1/11/10.
Now, add up your contributions to TSP for the last FIVE PPDs in 2007, all of 2008 and 2009. For 2008 and 2009 it's easy, just look at your last pay stub. For 2007, just add the last 5. 2010 won't have any yet as of 1/11/10.
Now, add your 5% government matching. For me it was easy...since I contribute 10%, I just added up my total and multiplied it by 1.5. Don't forget to add up any TSP loan repayments you have outstanding...as they were paid back into your account and count.
Now, add your contribution total to your 10/9/07 balance for your "if I got out at the top" balance. Compare to your balance as of 1/11/10. (NOTE: this is not exact, as it does not take into account the growth of you contributions). BUT...if we all do it the same way, the results should be comparable.
I bet there are just a handful of lucky souls who are positive. I am a bit disappointed in my number. I am -11 % since the market top. That is still a good chunk better than the buy and holder...and consider that I have been sitting in G since S&P 780! You can't win em all...but if you can beat the market, then you're doing well.