I read some methods but they're not accurate. They use the Archimedes principle and they assume uniform body density which of course is far from true. Others are silly like this one:

Take a knife then remove your head.
Place it on some scale
Take the reading
re-attach your head.

I'm looking for some ingenious way of doing this accurately without having to lie on your back  and put your head on a scale which isn't a good idea.

Get someone to relax their neck as much as possible, stabilize their torso, then punch them in the head with a calibrated fist and measure the initial acceleration.  Apply $\vec F=m \vec a$.