Suck a Girls Nipples
The nipples of girls breasts can be very sensitive and it may feel good for them to be stimulated. Before you get in between a girl's breasts and start giving her breast sucking tell you may want to tell her how much you enjoy them.
Some girls may be self-conscious about their appearance, so don't hesitate to let her know how perfect they are in your eyes. Some women may feel that their own are too small, too big, misshapen, uneven, etc, so sometimes it's nice to hear that someone else appreciates them just the way they are. Also remind her how much you adore her as a person, in general, so it's not simply about her being an object of sexual attraction.
One way to start is when a woman takes off her shirt, turn her around and gently kiss her on the back of the neck. The neck is an extremely erogenous zone, too, so this will turn her on even more. If she has a bra on slowly unhook it and help her take it off. Some girls find it sexy when a guy takes it off her.
Some girls enjoy their nipples being squeezed, but remember to pay attention to her body language because it may hurt, or if her nipples aren't sensitive at all, it may bore her to death that you're focusing on that area. Start with kisses around the area (working down from the neck) and then lightly give her breasts a tongue bath. Run your tongue slowly up and down her cleavage. Than slowly work your way towards her nipples.
First lick around the nipple. From here, you may want to actually suck on the nipple or you may choose to flick your tongue lightly up and down. Again, pay attention to her response so you know whether to continue or move on. Because you only have one mouth, while kissing, licking, and sucking one, you can fondle the other with your hand. You might move back up to her neck, shoulders, or mouth to continue giving those areas attention, too.
You might want to try playing with some nipple clips. You may also move on to oral, fingering her, etc or do any of the above during other sexual acts. Not all girls (or guys) have really sensitive nipples, though, so if she doesn't respond like you expected, don't automatically assume that you're doing something wrong. Just enjoy and move on to something else that you both like. Don't be afraid to ask her what it feels like so you can better adjust.