I like to ease my partners into things by playing with their ass and pussy at the same time. Ass stimulation might feel strange at first, but once they get off a few times while having it played with it's smooth sailing. I will never forget the reaction I got from my current girlfriend when I brought her to climax by licking her clit while having a casual finger in her ass. As soon as she started to cum I stopped with my mouth and just continued with my finger and she just kept on shaking and shouting until she couldn't take it anymore.

Just make sure to be careful with cross-contamination. If you are licking her ass you can't just go back to the pussy without risking transference of bacteria. I especially like eating my girls ass while she is on all fours, but I gotta be careful that not too much saliva is dripping down into her pussy or else it can increase risk of a UTI.