One of my favorite ways to design is with the use of "floral frogs." You can use the larger ones at the bottom of your favorite vase as a floral foam alternative OR you can place them on a more shallow plate for a less vessel-heavy feel. The plate should be able to hold enough water that the bottom of the stems can still be submerged. Many people use sticky tack to adhere their floral frogs to the dish. If you do it that way all surfaces must be REALLY dry and you must apply a circle of sticky tack along the bottom edge of the floral frog, that way it is totally sealed in. You press it against the clean, dry dish and TWIST. This really secures it. 

All of that to say, I like to gorilla glue mine to the dish and just use it as a floral frog dish permanently. This allows me to add really long stems and the weight of the dramatic length never pulls the sticky tack up from the dish. Below you can see a few photos of a floral frog design I did upon moving to Portland!