As far as I know, no position is any better than any other.
To improve your chances of conceiving you can:
1) be very healthy - eat well, drink lots of water, exercise.
2) keep track of when you're ovulating. If you have a regular cycle, this is the midpoint of the cycle. You can also take your basal temperature each day to more scientifically track this, or use an ovulation predictor kit from the pharmacy. Generally they say that you are fertile between day 8 and day 19 of your cycle (based on a 28 day cycle).
3) have lots of sex on your fertile days!