I'll tell you what i discovered in 3 years of pleasing oralsex with my gf (it's stll improving).
There are 2 ways to lick: teasing and "serious"
1) Teasing mode is soft, you lick all around her labias, suck the area between her legs and her vagina, gently pass your tongue on her labias and on her clit, but not often. Use your hands on the labias when you're licking the clit and on her leg when you're licking the entire vagina.
2) Then, when she begins to breathe very heavily start licking deeper, pushing your tongue between the great labiass to reach the clit.
Lick CIRCULARLY keeping your head pushed towards her vagina, and sometimes (ie: when you lose your position) suck gently her clit or her hole.
She will come strongly, often pulling your head towards her
Also try 69 and reversed 69, girls really love doing oral when they receive, it gives to them stronger felings!
Hope That Helps