A DIY vinegar rinse can be a straight-haired girl's best friend. It strips away heavy buildup (from conditioners, products, whatever) that weighs hair down, making it look lifeless and flat. Once you clean that stuff off the hair, it can bounce again.
Add about a tablespoon of vinegar to one cup of water (double the amounts if you have long hair). Get your hair wet in the shower, and pour the vinegar rinse all over. Run it through with your fingers, from roots (where it may also help eliminate dry scalp) to tips. Leave it in for several minutes, then rinse -- preferably lightly. Leaving a small amount of the vinegar in your hair can help prevent tangling.
You may smell a bit like vinegar while the hair's wet, but the smell will dissipate as it dries.
You can do this a few times a week. If you notice a drying effect, cut back.
Up next, an ingredient your straight hair doesn't want.