You should be weighing yourself every day.  We realize that weight fluctuates from day to day and you shouldn't expect the scale to be lower every single day, even if you ate perfectly the day before.  However, we are looking for trends and having daily measurements helps us do that more accurately.
[caption id="attachment_230" align="alignnone" width="300"] EatSmart CalPal Digital Bathroom Scale[/caption]
Here are some best practices:
Weigh yourself on the same scale every day at the same time (in the morning before having any food or drink)
Log your weight into MyFitnessPal on the Progress tab each day, entering your weight in tenths of pounds (ex: 150.4)
Once a week on Monday morning, we suggest taking a Progress Photo as part of your weight log
If you are traveling, don't weigh yourself until you get back to your home scale and routine
Don't be afraid of the scale - it is only a tool, and whether it goes up or down on a daily basis shouldn't reflect your mood for the day - it's just a number!