I have the same problem-it's really tough to measure oneself accurately! I use a scale that does bodyfat percentages, and although it's not as good as measurements might be, I find that it's a reasonable alternative.
Flutestarkiss9's advise seems to be right on the money if you do want the measurements-deliberately pick a method of doing the measurements that you can stick with every week, and stick with that method. It doesn't really matter it is, so long as its consistent!