Thicker wire, or litz wire. Litz wire is made of multiple strands of very small insulated wire, usually wrapped with some kind of fabric strands. It is NOT the same as stranded wire.
The insulation melts away with the heat of a soldering iron, so you just twist and solder the ends as if it were normal stranded wire.
As you go higher in frequency, the current becomes constrained increasingly in just the skin of the conductor. So larger wire gives you more surface area, but a lot of small wires can give you a -lot- more surface area in a smaller total diameter.
But if the strands aren't insulated from each other, then it acts like a single wire with grooves cut in it, making it worse than a single piece of solid wire.
Many Qi charging coils are would of Litz wire.
