Crossed legs are sexiest when your toes are pointed, and you aren't actually putting the weight of the top leg on the weight of the bottom.
(Heels are your cheat to pointed toes!)
Arch your back and sit upright. This is really great advice for ANY pose, but it's especially important when sitting to keep you from looking curve-less and short.
(Highly arched back makes her leaner, with perkier boobs. Pointed toes flexes her legs and makes them more taut and sexy. By sitting upright and doing these things, she's making a virtual cascade of curves...yay!)
(What not to do: flat feet compact her legs, leaning forward destroys any feminine curves, about the only bonus is that she has her neck elongated and her arms aren't merely hanging.)
Let your arms be active in the shot- dangle them, or play with your hair, or touch your body....dead arms are not sexy.
(Arms as props, and clothing! Also, arched back braced on her back arm. Well done!)
Be as dynamic as possible. Sitting normally will compact you and make you look less feminine. Try sitting backward on the chair, or sideways, or any way you can think of that lets you interact with the chair and be curvy.
(Although the arm crossed in front shortens her torso and blocks her breasts, she's posing dynamically, with crossed leg mirroring crossed arm and tilted head, and she's got great eye contact.)