That's a pretty broad question.
In general:
- Higher nic = more TH
- Higher voltage = more TH
- PG gives more TH than VG (but less vapor)
I have also heard things like vodka, ginger, even Tabasco sauce in the
juice bumps the TH. However, all these kinda muck up the flavor for me. So I use the first
three options listed to get my desired throat hit.