Lately I found some great variations of the arms above head position: the arms is not stretched fully, meaning she can move quite a lot, but is still vulnerable ofcourse.
Then, for intense periods I will stretch her out, if I am in front her by putting an arm around her hips and pulling her towards me while tickling with the other hand, or if I am behind her by hugging her tight with one arm, the hand taking a firm grip on her breast, and tickling with the other hand.
*Feeling* her squirm and shake with laughter, rather than just seeing it is really... I am sure you get it ;-)
Another little thing I recommend, esp if you are taller than the lee, is to release her, order her to bring a chair and then tie her up again. Its so nice to see her standing in front of you waiting for more tickling while you sit comfy in the chair she had to bring! 🙂 If she doesn't see the irony and humiliation make sure to tell her! Also the trick about about pulling her closer with an arm around her hips and now forcing her to stand with one leg on each side of the you & the chair is ... nice 🙂)