Going into labour with a positive frame of mind really helps, trust your body and concerntrate on remembering that you will soon be welcoming your little one into the world. Two out of my three births were without pain killers, not because they were easier, but because I found I felt more in control and active without them. It is a good idea to have your support person remind you of your wishes, or if you don't have a support person write it on your hand or a piece of paper. Having said all that don't be to hard on yourself if you change your mind.
All the best for a great birthing experience