The best glue for me at the hairline is superstick.

Used for 1/2 cm at the front hairline. I used to use at least 3 coats, using a hairdryer on medium setting for 1 min inbetween.

When the piece on your forehead, use a comb to mark the indentations on your scalp. These marks make it easy to act as a border for the glue.