During doggy style reach around and massage them. Massaging them anytime actually is great. Lick and suck, just don't be obnoxious about it. Like don't sit and suck on her nipples for five minutes straight. Mostly just tease. Like caress them, then flick your tongue on her nipples, caress again, then suck a little. Just vary it. Don't keep doing the same thing or it gets really annoying and uncomfortable.