If only we knew. Many of us have done it, but love comes and goes like the wind. There is simply no way to guarantee it will last. Two people are in a volatile market, selling themselves to the highest bidder. What are the odds that they are both the highest bidder?
For starters though:
1) Be in great shape.
2) Have great chemistry with her.
3) Meet her when she is ovulating.
4) Have sex with her ASAP.
5) Make her orgasm.
6) Be a challenge to maintain her.
If you can pull this off, you will most likely have her love for a limited time. One day it will vanish in an instant. Perhaps because her love faded, and perhaps because your own did.