One thing I used to do was stash all my gold in my bank alts guild bank. Every time I would pass a mail box on any one of my characters, I would send all but 100G to my bank alt. Every time I was on my bank alt, I would put all but 100G of his in the guild bank.
I was making gold at a fairly good rate like most people and like most people I was spending it just as fast. Getting that gold out of sight and out of mind slowed down how fast I spent it. It also limited how much I would spend at a time. Any major purchase would require me to relog onto my banker and withdraw the gold.
Once I started saving it away I also put goals on that gold. If I knew I was saving for a Nobles deck and needed 7,000G (at the time), I would have to think about that when I pulled the gold from that account. In the long run I spent less and reached my goal faster.
Now that I move lots of gold I do keep more gold on a few of my characters. I need quick gold to make bulk purchases some times. Those chars that buy stuff have about 2k gold on them. My main now keeps about 10K on him incase I want to buy loot from someone in a raid. Everyone else sits under 300-400G when I log them off.