When I'm loading up a fresh protank, or changing the head, I put a few drops of juice in the wick area.
After filling and assembly, I'll take a few large drags on the mouthpiece to prime the wick without firing the device.
Once I can start to taste the juice, I'll fire in short bursts to make sure it's vaping.
You can also let it sit and soak a while, but I never have the patience.