One tip from me, brush your teeth first if you’re really going to go at it. I hate when my husband comes at me with bad breath
This thread has some really good tongue techniques It’s more mouth on penis/vulva than mouth on mouth/boobs, but it could give you some technique ideas. One of the tips I liked is to keep using your hands while your mouth and tongue are busy, so if you’re kissing, you could run your hand through their hair, or while you’re sucking on a nipple, stroke your hand across their back or hips
This thread has some really good tongue techniques 
It’s more mouth on penis/vulva than mouth on mouth/boobs, but it could give you some technique ideas. One of the tips I liked is to keep using your hands while your mouth and tongue are busy, so if you’re kissing, you could run your hand through their hair, or while you’re sucking on a nipple, stroke your hand across their back or hips 