Toothpicks work well to combine pieces of the DNA model. The two different colors of toothpicks can be used to represent the phosphate and hydrogen bonds or a single color used to connect one piece to another with no representative significance. In addition, multicolored toothpicks can represent the nucleotides and sugars, while colored Styrofoam balls represent the phosphate and hydrogen bonds.
Paperclips come in a variety of sizes and colors and fit together easily for a quick DNA model. Because assembling the paperclips does not deform or weaken the components, students can reuse the same batch of paperclips year after year without difficulty. The only concern would be the loss of paperclips. However, only one or two new boxes of paperclips would need to be bought every few years, making paperclips highly cost effective in comparison with Styrofoam balls or pipe cleaners, which become less stable over time.
Foam balls of 1/2 to 1 inch in diameter can be bought at any craft store. Used to represent the nucleotides, sugars and often phosphates of the DNA model, these balls must be painted different colors before using them to build a model. The balls are then connected together with toothpicks. Once the model is no longer need, students pull apart each component and store them in boxes or bags for the next time. Over time, the paint will wear off and the foam balls will begin to disintegrate from too many punctures, but they should retain their integrity long enough to pay for their initial cost.
While a considerable initial investment, DNA model puzzles are probably the best long-term investments. These educational toys are available online and in specialty shops and contain hard plastic pieces. If carefully assembled, disassembled and stored, a plastic DNA model could last indefinitely. In addition, these models are designed to fit together in such a way that the model naturally assembles into DNA's characteristic double-helical twist, so the students do not have to twist and hold the model to examine this important aspect of DNA's structure.
If you have a loose definition of reusable and your budget allows, gummy or soft candies make interesting and tasty building blocks. Jellybeans, gummy bears, jujubes, Junior Mints and other pliable candies piece together with toothpicks to form a DNA model. When the students have successfully completed the project, the components can be reused as a sweet treat. Add an educational aspect to the dismantling of the model by only allowing the students to pull off a piece of candy if they answer questions correctly.