Some have tried adding a little Everclear. In general, the higher the nicotine mg level, the harder the TH. Also, some flavors allow more TH. Dormouse is right though; PG has a harsher TH.
So, to answer your question, there is nothing you can do to increase TH without losing some flavor, but adding a little pure PG or Everclear may be worth a slight flavor decrease.
I've personally noticed that pure VG juices tend to need a little more nicotine than PG mixes for a desirable TH. For instance, if you normally like 12mg PG juice, try an 18mg VG juice. That's just my experience.
Good luck