Some of the best salads don’t include any firm vegetables, and this can be problematic when it comes to layering your salad dressing at the bottom of the jar. Fruits, for example, will soak up the vinegar in dressing. At least one company sells reusable cups that fit into the tops of mason jars, providing one solution. But for a quick and inexpensive fix, all you need is parchment paper, or even plastic wrap.
Source: Julia Mirabella