December 2007 net worth update

My net worth increase by $3,500 over November. My YTD rate of return for 2007 was 10%. Um. Boo. One of the things I need to do soon is readjust my 401k assets. I could do the target date fund my employer set up, but you have to pay a fee on top of the fees for the funds themselves. I'm thinking, why can't I mimic the same thing myself?