In latex fetishism, a dressing aid is something that makes it easier to put on a tight latex garment. The best-known is talcum powder, which creates a layer between the skin and the latex. Nowadays, more and more people are using lubricants such as silicone oil. If you apply silicone oil to your skin or to the latex, you can just slip into your garment.
Care must be used in choosing a dressing aid, since some (especially oil-based ones) can damage latex.