Hallo lilmysterious,
from my point of view there are a lot of "best" positions a sweet girls armpits can be tickled in.
I hafe tried many positions and my individually all-time-favorite way to tie someome up is :Fix her arms behind the head (in any kind you like) and the forearms bent backwards, that makes her more immobile and she feels totally helpless, ... it's also an psychological aspect .
I don't know if you knew, being ticklish is only happen in the head and nowhere else, its medically proved.
Tools and Medium : A fork (yes for eating !!) if it's not to sharp and paint-brushes and the classics : fingers and feathers .
Last but not least my very best tip is : Use Oil !!!!! Oil can be an intermediary medium between you and her sensitive skin.
This makes the tickling more worse, especially if your fingers are the tool. It's a great feeling for you to.
A priceless advantage is : Your slippery dancing fingers will almos fly over her sensitive skin and you can crawl her pits as fondly as a little kitty. You never will have the same result without oil. You will definitively see it on her reactions.
That this way of tickling is almost(?) unbearable i can tell from first hand because my ex-g.f. (first pic) did it to me too one day .
Ohhh, I had almost fogotten to say : Use conventional vegetable oil only, because if you feel like licking her pits or to tickle her pits or other delicate bodyparts with the tip of your tongue babyoil really tasts not that good !!! (grin)
I've posted some pics to show you "my best way to tie someone up".
....hope you like them.
Greetings from Germany :super_hap
Lil-Teaser
P.S.:I whish I could see her reactions and your face if you try one of these out !!!