For me, the crease of the leg and thigh is pretty sensitive, as are the tops.of the thighs. The sit spot isn't too sensitive and does get built up quite a bit. For a long spanking that bring some to tears I like a nice solid tanning.all.over, but some really hard, sharp whacks on these more sensitive spots. Especially if I am surprised by them. For me.. the initial surprise and then the tap tap and an intentional strike there helps get me to the point of tears.
But it also heavily depends on the mindset. So what is going on during the spanking? Where is your mind at? You can also try taking breaks during the spanking, frequently to help stop or prevent the endorphins early on.