I use two sets of 1/2 inch quick disconnects I got from McMaster Carr. One set is positioned at the lowest point in my loop, just before my pump intake. The other set I attach to lengths of tube that reach far enough to get to a bucket. When It's time to flush I just pull the cap on my res, attach the second set of valve's and the coolant spills out.
This approach is so easy that I've started flushing my loop with 2 or more gallons of plain distilled before I toss in any additives.