The perfumers apprentice liquid menthol.
http://shop.perfumersapprentice.com/p-6674-menthol-liquid-.aspx
Use it sparingly, a little goes a long way. Start very low, around 2%(0.2 ml per 10ml), shake, test, add 1%increments until you are happy.
It is also handy to add a bit of extra menthol punch to a purchased eliquid, but be gentle.
http://shop.perfumersapprentice.com/p-6674-menthol-liquid-.aspx
Use it sparingly, a little goes a long way. Start very low, around 2%(0.2 ml per 10ml), shake, test, add 1%increments until you are happy.
It is also handy to add a bit of extra menthol punch to a purchased eliquid, but be gentle.