1.) A Good Petticoat: What I like to call the foundation of a lolita outfit! A good Petti with the right shape is often what separates the beginner lolita from the matured lolita.
A good thing to keep in mind when you're shopping is to look for pieces that can tighten at the waist(lace-up back, shirring, waist tied bow). So, if you buy a slightly bigger size you can still look great by showing your waist to be the smaller part of your upper body.
Same thing with mainstream clothing! It's not about squeezing into a size 2. It's about what size fits and flatters.