My gf and I are both very similar in size, and she doesn't that much less than me. This makes her a formidable tickle-fight opponent. So, naturally, through tickle-rassling, we end up finding all kinds of spontaneous positions.
My new fav was when I had her on the floor ,on her back. I was sitting in kneeling position on my knees, no top of her, half-sitting on her chest (the weight was on my knees, so I could control how much I put on her body). Each leg was pinning one of her arms up, so she was immobile.
Basically I had full access to her pits, ribs, and tummy, and she could not see anything or do anything. I think having her vision blocked by my body made it worse for her as she had no idea what I was going to tickle next xD