I measure every week, and I measure most everywhere. But I only measure the right side, because I'm just looking to see the change.
Weekly is too often really; some parts don't change that fast, and some parts you will just a see a lot of up and down at that frequency.
Some parts are so hard to measure consistently. For example, my waist is easy; it's well-defined. But I also measure at the level of my belly button (which is quite a bit lower than my waist; my waist is practically up under my girls) to follow how the belly is going down. My consistency on that sucks. Too much curve

.
For my upper/lower arms/legs, I have little landmarks that help me measure in the same place each time, like a freckle or something. My arms I flex to see the biggest part of the biceps, place the measuring tape, then measure relaxed.
It's all probably overkill, but I like see all the changes over time. There's always *something* that goes down, even if just an eight of an inch, even when the scale stays the same or goes up a little.