When it comes to dahlias, it's all about the vase. I found that in my wide-necked jugs, they just fall all over the place and hang down, as many of my dahlias have such heavy flowers.
So, a narrow-necked vase is the way to go. You can often find them in charity or second-hand shops; they're kind of a relic of a bygone era, I think.
Let me know what finds you have and what arragements you've managed to make! I'd love to hear from you.