Depending on the material you can keep it taught between the front and back of the feed dogs and it will almost pull the material such that there isn't a curve, and its just like sewing an uncurved piece. Utilizing all the above provided info will help as well -
note, this may not work with ALL materials, only those that have a certain amount of stretch to them.
oh, and depending on the amount of stretch, you might end up with what looks like the material is trying to "roll" into the curve - this just means you pulled a bit too tight.
at least all of this is true in my experiences - ymmv