I find that I hold sodium like crazy, so if I've had a bad sodium day (Chinese food, or a pre-packaged lunch, or something processed) the scale won't budge for days. I also find that if I only weigh myself a couple of times a week, it helps get rid of those ARGH moments because it's likely the scale won't be the same after a few days.
Plateaus and non-budging scales are beasts, however. My longest plateau was four months and it was awful. Just keep on keeping on! You know you're doing the right thing, eventually the scale will reward you.
Find other ways to track, too! Measurements help.
Female, 30 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 5/19/14 = 179.6 (net: -35.6)
Current mini-goal: 175 lbs.
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."