Bitter greens and greens in general are on my 'favorites' list. I've learned from some different people I've met and settled on my favorite ways to cook them.
I always, like Norm, cut the entire stem out of the leaf ~ all the way to the top. I fold the leaves in half and slice along the stem, cutting so I end up with two halves of the leaf. I read someplace that the sulphur (bitterness) is produced in the stem so it makes sense to me to get rid of it at the source. If I'm cooking collards or broccoli rabe, which are IMO more bitter than most, I will sometimes par cook them in salted water and then into a sink full of cold water to stop the cooking and rinse really well. Then they get drained and I'll continue cooking with the rest of the ingredients. That first cooking in the salted water helps get rid of that sulphur.
Then, for most, I'll start with some bacon to render down, remove & reserve the browned pieces but leave all the fat to flavor the greens. I'll add onion, red pepper flakes and the greens, very little salt because it seems they retain a lot of the salty flavor and it's really easy to overdo it. Then I cook in homemade ham stock if I have it or boxed chicken stock. Now, here is the #1 'secret'.....BROWN sugar. An old wrinkled Southern woman whispered it to me many years ago....."I add a handful of brown sugar to my greens and no one can figure out why MINE are the best", she said. She only told me because she thought I was a northerner and would NEVER continue making greens so her secret would be safe with me. (Since I like the bitterness, I only add about 1 Tbsp. of brown sugar ~ that and the fact that I'm cooking about 1/3 of what a 'normal' Southerner cooks in the collard pot.) There is some way that the molasses in brown sugar counteracts the sulphur of the greens.....I think it's some sort of science ~ could be something I heard 'my man' Jacques mention once or twice. You can also add a tsp. or so of regular molasses during the cooking if you think your greens taste too bitter.
But, it works. For me, anyway. I will let the greens cook down, test them, maybe add more stock a bit at a time, and finally, the reserved bacon goes back in and given enough time to soften back up.
One of my favorite ways to use leftover greens is to add them to a can of cannolinni beans and let the greens be a 'flavoring agent' instead of just eating them plain.
And, of course, my #1 way to eat greens is for breakfast....you've all seen it many times: