You spend 20 minutes wrestling your toddler into the tub, plus an extra five enduring the ear-splitting screams that arise when a single, clean drop of water runs into her eye. Finally, she's in her pajamas; the end of your nightly marathon is in sight! Then, as if she were purposely trying to push you off the narrow cliff of sanity you pride yourself on maintaining, she undresses herself -- pajamas gone! Now you have expend what remaining energy you have chasing her around the house trying to replace them. Believe it or not, it doesn't have to be this way.
Let you toddler "air dry" in the buff after her bath. As long as she stays inside the house, or in her room if you have company, it's okay to grant her a period of clothing-free time. This reduces the time she has to undress herself -- mainly because she's not wearing anything. The developing sensory systems of toddlers often make your little one opposed to wearing clothing. And getting dressed when you have damp skin can feel especially irritating.
Explain that before she gets into bed, you're going to help dress her in pajamas, but that she can complete her bedtime routine -- brushing her teeth, washing her face, combing her hair -- while she's wearing only a diaper or underwear. This allows her to enjoy the freedom of being naked but also tells her when to expect pajamas. If she's determined to take off her diaper or underwear, just wait until bedtime to dress her.
Dress her in "escape-proof" pajamas right before she lies down in bed for story time. These one-piece, footie pajamas have the zipper in the back, along with an additional closure flap and double snap that protects the zipper from her fumbling little hands. By the time she's lying down in bed and listening to you read, her attention will hopefully be on the book, not on undressing herself. Also, undressing is nearly impossible without sitting up in bed, which is exactly why you should insist that she remain lying down.