Hiya, I have string jointed arms and legs a couple of times in the past but never heads, so I am not sure on how to do a head.
But for arms, I don't tie any knots in the thread but start on the underside of the first arm, put the needle through the arm at the top and leave a length of thread on the underside, then back through the arm and through the body to the other arm. Go through the other arm underside to right side and back through leaving enough thread to hold the arm in place, back through the body and then you should end up with two bits of thread next to each other, one in the first arm and one through the body, tie them together and pull as tight as you need to tighten both arms.
Hope this makes sense, it does seem to tighten both limbs this way.
The is the second larger bear i have made, this time out of Schulte sandy and brown mohair. Designed and hand sewn by me, i have not named him as i thought it would be nice for his new family to be able to name him. More info on my website http://www.barnetbybears.co.uk/?page_id=19
I have just finished this handsome guy, he is 22 inches, larger than the bears i normally make, he is made of gorgeous Schulte golden mohair, which looks darker in the house but in the sunlight really shines as the pics show. He is available on my website now.http://www.barnetbybears.co.uk/?page_id=19