Mostly by keeping the receipts and recording my expenses. That way, I can pinpoint how much I spend on a weekly basis.

I usually save money by getting what I need and find cheaper prices. If you just arrive at a new city, spend some time checking out all the possible stores. If you're going to stay around for a while, you should try memorising all the cheap places.

After that...mostly trial and error, so you can figure out the right balance for yourself.