Making perfect grapefruit sections isn’t as difficult as you may think…in fact, it’s easy!
Here’s how to do it!
Using a sharp knife, first cut off both ends of the fruit. Then cut away the rind and pith by following the curve of the fruit with your knife.
Lay the fruit on its side...
Insert the knife just to the right of one of the membranes and cut to the center of the fruit...
After the knife has reached the center of the fruit, twist the knife so that the cutting edge is now pointing out towards the outer edge of the fruit...
Your knife should now be cutting towards the outer edge of the fruit...keep cutting until the fruit section is released (You are now just basically resting your knife on the opposite membrane from where you started and scraping the fruit section off this portion of membrane)...
If done properly, the grapefruit section will just fall out of the fruit! 
Sectioning a grapefruit in this manner reduces waste because there is less cutting around the membranes...you aren't making the 2 "traditional" cuts. By just making 1 cut and then allowing your knife to follow the natural curve of the membrane makes this way of sectioning a grapefruit easier, quicker and you will be left with more fruit to enjoy!