Keep your arrangements simple. People tend to put so much stuff in, and their house ends up looking like a Mafia funeral parlor. Do ten sunflowers in a cluster. And everyone loves cymbidium orchids, which aren't that expensive and can last for three weeks. Or you can always do a monochromatic arrangement of different flowers, all together, in a bright color.
Be creative with your containers. I've done arrangements in a soup tureen, a martini glass. Look in your cupboard.
If you're buying flowers from your local supermarket, stick with roses. They're the most popular choice, so there's lots of turnover of stock. Just make sure the blossoms are a little bit hard in the center—that means they're still fresh.