Shaping Slip - this is literally a life savior, and a perfect piece to go under dresses for that extra level of security. I wear quite a bit of white and lace dresses, which often times can be a bit too see-through for my taste, but having a slip handy easily solves that problem. Normally, I tend to go for body-hugging styles since they work equally well under form-fitting pieces and flowy ones, fit like second skin, and are completely undetectable under clothing.
Half Slip - since shaping slip doesn't always work with skirts (in case I'm wearing a skirt with a crop top for example), I lot of times, half slip is what I opt for when I wear skirts. A great layering piece, normally, I look for ones that have a slit in the back so I don't feel constrained when walking around as I've struggled in the past with slips that were simply impossible to move around in.