Add a certain number of points to everyone's grade. For example, if the highest grade earned was a 95 out of 100, the teacher could add five points to every student's grade for the curve.
Reduce the number of points the assignment was worth. For example, if the test you created had several incorrect or too difficult questions, those questions could be dropped and the test could be adjusted to a basis of 95 instead of 100 points.
Drop the lowest assignment grade each student has earned. This action will help curve everyone's grade slightly up and will give students hope that even if they have one bad assignment or bad day, they will be able to recover.
Decide if, instead of modified curving of individual assignments, you would prefer standard curving of quarter and/or semester grades. If only one student earns an A and you feel that perhaps three should have earned an A, then everyone's grade could be rounded up an equal number of points in order to have those two other A-worthy students finish with A's.