Drooping comes naturally to the tulip and can be part of its charm -- even tulips in the garden are weighed down by their full blossoms. And besides bending to gravity, tulips tend to lean toward sunlight, adding to the droop. Still, there are steps you can take to improve their posture.
First, only buy the freshest tulips, with bright-green leaves and crisp stems that squeak when touched. Then, instead of displaying them right away, condition them overnight in tall container filled with clean, cold water. If they were wrapped in paper when you bought them, leave this on to provide further support. Create your arrangement in a sufficiently tall vase and change its water daily.
If you are still unsatisfied with the rectitude of the stems, wiring is a foolproof solution. Take a length of 20-gauge florist's wire and delicately slide one end up into the stem, just below the bloom. Wrap the remaining wire down the stem in a loose spiral, clipping off any excess. Now you can adjust the flowers to an exact straightness that they will hold indefinitely.