As gravity pulls the bloom down, the surface of the foam becomes a pivot point to the stem. If the stem is a little short, add a pick extending beyond the end of the stem, to give it more security. Fit a square block into a round hole. Cut all stems at a sharp angle. Use floral picks for various tricks.
Final Tips Use just enough foam to hold the number of stems necessary for the design.
Two inches of foam extending above the edge should be sufficient for a simple line design. Carefully gauge the length and position of stems so you just poke once. In modern design, the foam may be visible, but should not be distracting. However, you can store wet, unused foam in a zip-lock bag in the refrigerator for later use. I've had designs built with floral foam travel 20 miles in a car without coming apart.