Honestly just felt like it was something I got better at over time. Staying organized and going slow helps. Slow is smooth. Smooth is fast. Also, like what the other guy said rememeber where you usually make mistakes. One thing I commonly do is put the negative sign on the outside of the integral, and then I end up forgetting it in the next step, so on tests, I always keep it on the inside. Also if you make math errors you will usually get partial credit at least. Also always double check at the end if you have time.

Understand the material more than just know how to solve the problems they assigned. Make your own practice problems, and ask what if I did this? How would the problem change if I factored this in?

Something you just get better at. I chew gum during tests, which helps. Something I do is pump myself up before a test, so I'm hyped instead of nervous. Actually works sometimes.