The most important thing you can do to a women in bed is to pay attention to her boobs, and here's how to make her nipples harder than rocks:
Nipples are the body's thermometer. They're brilliant indicators of whether your moves in the sack are turning your girl on or boring her to death. Obviously in some cases such as inverted nipples, this isn't the case, but for the majority of women, nipple play is a great source of foreplay.
Trace your finger slowly around your lover's nipple and you should get an immediate reaction, where her nipple rises and hardens. The more you play around with her nipples using different stimulants, the greater her reaction will be.
To get rock hard nipples, try out some of these stimulants:
Lick, then blow on your lover's nipples, then repeat the action until hard
Trail ice cubes over and around your lover's nipples
Lightly bite and flick them – it's a major turn-on!
Finally, roll her nipples gently between your thumb and finger with a drop of lube