How are you basting the quilt? That can make a difference. I prefer spray basting because it holds all the layers together better without shifting.
Starting in the middle is great for hand quilting. For machine quilting, I much prefer to start at an edge and finish at an edge. Some people bury their threads that are in the middle of the quilt. Leah Day has a great tutorial on how to use a "cheater" needle to do this more quickly. It's probably more common for machine quilters to secure the end of the stitching line with 3 or 4 very small stitches, then cut threads close.
A walking foot can be used to quilt gentle curves.
I experiment with stitch length/width as a lot depends on the batting and how thick the sandwich is. The thicker the batting, the longer the stitch that you want to use. It's a matter of seeing what looks best to you.
To prevent puckers, I like to heavily starch the backing before layering. This is especially helpful if you are crosshatching or doing other quilting lines that will cross each other. (Spray starching the top helps too.)