I'm also a serial weigher...
If you're going to do it, then try and get as close as poss to the same time and under the same conditions each day. Any change in clothing or the amount you drink can change it by 3 or 4 pounds! I reckon the easiest is to weigh first thing in the morning (after going to the loo but before you have any drinks).
It will fluctuate during the day and you'll usually retain water (and therefore weight) around your time of the month... As Dee says - even then, you'll sometimes see weird blips. I usually see a drop (my electronic scales only measure in full pounds) every 2 or 3 days, but sometimes I can stay the same almost all week and then drop loads overnight. A couple of days ago I thought I'd gained a pound, but an hour later I'd lost a pound! So, don't get too hung up on it and keep looking at the big picture too...
Mx