I recently went through a 6-week plateau (which is still a lot shorter than some). I was stuck at 314 for so long that I began to hate that number on a very personal level! I still resent the number 4 just a bit...
The main advice I can give you is don't let frustration get to you. Continue with the good work, but this might be a time to shift your focus from the scale to other measures: how you're feeling, how well you're perfroming during exercise, how well you're sticking to your plan, etc. The scale is only one of many ways to measure your progress.
Also, I really have found that switching around my exercise is important. I go to the gym, and was doing just the treadmill for my cardio. Since I was increasing the difficulty settings on a regular basis, I thought I was doing great. However, I tried the eliptical one day and found that I could barely do that on the lowest setting! Now I switch between treadmill, bike, and eliptical. If you've been walking, try riding a bike for a while, or vise-versa, or try a new sport, or swim.... just something that will use your musles in a new way.
Number one, though, is just don't give up... things are happening, they just don't involve the scale. Good Luck!