Measure the same as you did the first time, then you'll be measuring with the same variables.
Don't measure too often either. If you are measure very regularly you wont notice much as you'll be measuring so much and you'll get sick of seeing nothing noticeable on the ruler. If you leave it a month or two then youll see the difference and it'll spur you on a lot more. It's like waiting for something, if you keep looking at the clock, it goes slowly, if you just leave it, time seems to go faster. The ruler is the clock, and a gain is what you are waiting for.