I don't have a night blooming cereus, I wish I did though! It is my understanding that they blossom off the old growths, so you don't want to be to excited to cut it back or cut back too much.

Maybe this will help:

Quote: cereus get to be big, very big cacti. this is their nature and pruning should be limited to side branches to keep the plant in proportion. if you cut a main branch it will scar and look terrible unless this is done at the base of the cactus. most likely where a cut is made new branches will form off of its sides.

if the plant gets to be just too big, i would suggest you cut side branches, let them air dry in shade during the warmth of a summer's week and then plant. they readily root and you will be able to grow multiple plants to replace the big, big momma. branches you want to root will sunburn on sides not previously exposed to the sun.

http://www.desert-tropicals.com/bbs/posts/3363.html