Walk as much as possible. You can save SO MUCH by not taking cabs. Plus, it is good exercise and you get a closer look at the culture.
If you can't walk, try taking the trains or a bus.
Choose an accommodation with breakfast included--and eat A LOT. This way a little snack in the afternoon could hold you over until dinner.
Research "free things to do" Ex. London, you could spend an ENTIRE DAY in the British Museum and not get bored.
Limit your souvenirs. The value of pictures and experiences will last longer than keychains.
Stay at a hostel--most are only $15-$30 bucks a night AND include breakfast.