Nothing sets the mood for a party better than flowers. For all the at-home parties I've done, I always have vase on the table. I think of it as a thank you to myself for organizing an event. And if it is a kiddie party it's a nod to the adults in attendance that I care about my home. Here are my tips for using flowers as decor at a birthday party.
If you are on a budget stick to a single flower bunched together. I can always find relatively cheap tulips here in NYC. I pick one color and buy a few bunches. At $7 a bunch it make a room come alive.
For kiddie parties, keep things colorful. My little girl is always thrilled to come pick out flowers. If there is time I take them to the farmers market. My July-daughter likes sunflowers--perfect!
Consider ordering online. Companies like 1800flowers.com have a whole section of birthday flowers from $24.99 (some with same-day deliver). Ordering ahead will save you time and effort in arranging.
Bring your own vase. If you are using a florist, drop off a favorite vase or other decorative item. Another idea for inspiration is to give your florist a copy of the invite so he/she can see the color scheme.
Get inspired. The blogosphere is full of talented designers. A quick search led me to Flower Duet.
Keep them fresh.Real Simple has a good list of tips.