I'm speaking as far as new rads go, which I still flush. First I run hot tap water thru for about 20-30 mins. I have a connection that screws onto faucet with a barb on it so that made it easy.
After that, fill it full with distilled water and half cup to a cup of vinegar. Top it off and let it sit for 20-30 minutes, not too long or the copper inside will start dissolving into the vinegar. Well, a little of that will happen anyway, when you dump the water out, it will have changed color slightly to a bluish tint, signifying dissolved copper. Some people won't use vinegar because of that, but a little like I described is ok.
After that just fill it half full with distilled and close both ports, then swish it around in there for a minute or so. Dump out water into a clear container so you can see what's in there, you'll likely see little specks. Repeat until its clean enough for you.