I've worn skin units and lace.

For me, the best is a white glue that dries clear. Basically, pick your brand. They all seem to work decent.

The only touch ups I ever need to do is the front 1/4 inch of the hairline. Basically every 4 days or so I have to touch it up. Not bad at all! The rest of the unit stays with a strong bond for 2 weeks or so.