Pants frustration

When I first started running (back in August 2011), one of the first things to happen in the first couple months was that I dropped a pant size.  Woohoo!  I was so excited - I bought a couple pairs of pants in the smaller size and was so thrilled.  Then, after about six months, I couldn't fit in those pants anymore.  I was back to the old size.  UGH.  So, I guess I assumed that with more running, more exercise overall, and better diet that I would be able to get back into those smaller-sized pants.  I run 20 miles a week.  I work out five days a week for at least 25 minutes at a time.  I eat a ton better now than I used to.  Those damn pants still.  don't.  fit.  Though I have lost 5 pounds since the beginning of the year, I'm still really frustrated that the scale moves so slowly (and sometimes rebounds!) downward, and that I'm not seeing much improvement in toning.  I refuse to go on one of those crazy drink-only or pill-only diets (that's ridiculous in the long term), so I guess...I don't know what to do.  I'm happy I'm in shape, I just want to see my damn weight go down!