i have a scale that measures bodyfat percentage as well as weight. I got it from costco for about $40, and it's great. I don't know how acurate the bodyfat percentage measurement is, but I don't care. What I care about is that it's consistant. I've tested it by stepping on it multiple times in a row and got the same reading each time. So it may not tell my my bodyfat percentage, it will tell me if i'm getting leaner or fatter, which is after all what we want to know right?
You might want to try one of these out.....it helps a little with the "muscle weighs more than fat" thing.
oh, and I always weigh myself at the same time of day so I don't mess up the numbers with intra-day fluctuations.