I do a few variations on this....

first off... i use white gas... lamp oil and isopropyl don't work as well. for both of these, use a lit wick.

one way, I place the burning wick on my tongue... raise the wick to vertical, insert fully into mouth, then remove. The slower you do this, the greater the dramatic effect. Now the shape of your mouth is key... a O shape gives less Oxygen, makes a sort of fire kiss, and burns longer... and 'Ahhh' shape gives bigger flame.

I really think the tongue is key for these tricks... you draw fuel onto your tongue, which sustains enough fuel to burn as a kiss.

the fire kiss I like involves inserting wick into said mouth, then closing your lips around the wick... you can extiguish or not extinguish as you like. I like to give a little suction as well. (overcome yer gag reflex) slide the wick out and keep your lips closed. now, there is fuel/fumes inside your mouth and you can keep your lips closed as long as you want... breath thru yer nose. When it's time... purse your lips and create a small opening and light the fumes. I try to very gently squeeze the empty space in my mouth closed or blow very gently. you can sustain the 'lamp' for as long as you got fuel/fumes.

I recommend the second trick after 60-90 seconds of burning... otherwise there is enough residual fuel where you can squeeze the wet fuel out and on your face.... facial burns suck.

happy dental bills!