I agree with Aphil. Adding weights to your routine, even if it means cutting out a little time of your cardio schedule, can be a great boost to your metabolism. And, it improves the appearance of your body significantly even if you stay at 140 for months.
With that said, nobody LIKES to plateau but all of us do at some point. Two things have helped me mentally push through when I've gotten stuck.
First, look at the overall picture. Do you plan on giving up healthy eating and exercise once you hit your magic number? If not, then the number on the scale really isn't all that important. Your main goal should be to live out the rest of your life healthily and your weight shouldn't stop you. Keep doing what you have been and the weight will eventually take care of itself.
Second, I remember why I care what the scale says in the first place. Vanity, of course. My health is my number one concern but, make no mistake, I want to look good too. So, when my weight plateaus I have one of two options. Stick it out knowing that eventually I will reach my goal albeit more slowly than I would like or give it all up, undo all of my hard work, and regain every pound. I am not happy with my current weight. But, I'd much rather be stuck at 153 for the rest of my life than ever be 214 pounds again. I don't look as good as I would like to right now but I look a heck of a lot better than I once did.
Don't give up. Try and mix up your food and exercise routine - it might jump start things but if it doesn't at least you won't be bored of doing the same old same old without seeing the results you want. Do your best to focus on something besides the scale - take your measurements, track your body fat percentage, try out some new healthy recipes, or gradually increasing the time and/or intensity of your workouts. Anything that will help keep your program fresh.