I weigh myself every morning when I first get up. It's part of the routine so that I can maintain near identical conditions. Yes, I go up and down between weighings but at least i can see the trend and the Wednesday morning weight doesn't come as a total shock. I see a definite weekly pattern - I control my diet better at work because I've no opportunity to snack. I'm at my lowest on a Saturday morning and my highest on a Monday or Tuesday. I can spot the effects of a big meal some 24 to 48 hours later.
Weighing daily gives me the motivation to continue. If the trend is on the downward slope I give myself a mental pat on the back and tell myself how well I'm doing and think what I'll manage if I can just continue. If it's level I can think about what I should be doing to get it sloping downwards again, and if it's up I give myself a boot up the arse and log a "must try harder". If I had to wait a week to find out if my efforts over the previous time period had been effective or not, I'd just give up at the first negative move.
The key thing which has helped me though, is not weighing myself, but weighing my food to control my portion size. It was a sobering thought to realise that my "50g" portion of muesli that I put out every morning was in fact, 200g+. Once I stop kidding myself, things tend to improve. I quite deliberately now eat off a smaller plate and use a smaller spoon and fork so a "plateful" isn't as much as it used to be, but it is still the same number of spoon/fork-fuls.