Butter. When I switched back from margarine it was night and day.
Quoted for Truth.
Check your margarine. I suspect someone grabbed a low-fat margarine instead of your regular stuff (since you said you’ve done this before successfully with margarine). It is indeed too high a water content - you’re steaming your sandwich as the water in the low-fat margarine evaporates up through the bread. Steaming is good for carrots and potstickers, but not so much for grilled cheese.
Or, is it possible that They just changed something important in the margarine making processes with the banning of transfats? Perhaps even regular margarine has issues now. In that case, what everyone else said: butter or bacon grease or mayo all work well. Got to keep your heat low on olive oil, but it will work too; I don’t like the flavor for grilled cheese.
It’s funny how an innocent list of good things turns into something so heinous and disgusting. 
I feel like such a philistine. All I do is put a slice of cheese between two slices of bread, put them in my sandwich maker, wait five minutes, and eat.
I learned that trick from this board years ago and it does, indeed, work quite well.
It works fine with a completely dry pan as well
I’m a little unclear on your technique for removing the sandwich when it’s done, unless you skipped the part of the recipe which calls for flinging the sandwich into the lava and heading into town for Mai Tais and mahi-mahi.
Jumping on the the ‘use an oil’ bandwagon, I’ve switched to unrefined sunflower oil and will never go back. In your local farmer’s/international market there may be a Russian section… the nutty goodness is a deep gold-colored liquid bearing a label with small writing containing a word that looks like HEPA-something-something.
Another vote for “use butter.” I also toast my bread in the toaster (lightly toasted) before putting the sandwich together. Make sure the frying pan is hot. The bread is already toasted so I just want the the cheese to melt ASAP.
Get the pan hot on about medium. Then add some olive oil to the pan. When it shimmers, put your sandwich in. You don’t want the heat too high, as any fat will brown food quickly, and you want the cheese to have a chance to melt. You could also try flavored (infused) olive oils like lemon or rosemary or others. The addition of some grated Parmesan cheese to the pan will make for a nice exterior crust on the sandwich.
I was going to say, ‘Sometimes I cover the pan, and sometimes I don’t.’ Then the first bit of caffeine found a crevice. I don’t cover grilled cheese sandwiches to melt the cheese before crisping the outside. I do cover grilled [something] and cheese sandwiches. For example, if I’m making a tuna melt I cover it so that the tuna gets hot.
I reserve butter for cooking, so I never have salted butter. Also, it’s kept in the fridge. Even if I left it out, it would still be more difficult to spread than butter.
Sometimes I’ll use California Olive Ranch olive oil, which gives the sandwich a nice ‘green’ taste. Trader Joe’s has a similar product that’s almost as good. Or there’s Trader Gioto’s olive oil. But if I use olive oil, it’s because I want the green olive flavour.
I like Fleischmann’s olive oil and sea salt margarine. I don’t care about the sea salt, but I reckon the olive oil might be just a little healthier than whatever other margarines are made of. And it spreads very nicely.
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FWIW, I have a dedicated sandwich pan. It’s a cast iron fajita pan I picked up at a yard sale about a decade ago for $2. It’s very nicely seasoned; slicker 'n snot. One of the best two bucks I’ve ever spend. 
I seem to recall using some leftover ghee to make a grilled cheese. Nice, nutty flavor.
You can always nuke it for a couple seconds. (And I also only have unsalted butter around the house.) That said, when I am not using the mayo trick, I just melt a bit of butter in a small cast iron pan and just put the grilled cheese in dry, then, when flipping, throw another pat of butter in and melt for the other side. This is the way my friend’s mother always made them (and she made the best grilled cheese sandwiches. My folks didn’t do grilled cheese.) I wasn’t even aware of the butter-the-outside-of-the-bread method until my SDMB days.
Exactly. My mom was a bread butterer, so that’s how I always did it. Then one day as I was tearing up a piece of bread with too cold butter, I had a revelation, and decided to butter the PAN. :smack: Wow, that was much easier!
I went nuts and used the waffle iron yesterday. Crispy goodness and many crevices. That was a good sandwich.
All this talk of pans and frying is boggling my mind. Don’t you use the grill (broiler?) to make cheese on toast?
For me, as an American, that’s a little different. While the name is a bit odd, to me, a “grilled cheese” needs to be cooked in a fat of some sort (preferably butter), and fried in a pan or a griddle. Perhaps “fried cheese sandwich” would be a better name. I’ve never actually seen a “grilled cheese” that is properly grilled (over fire). If it’s not buttered and fried then, to me, that’s not a “grilled cheese,” that’s just a toasted cheese sandwich. Perhaps the “grilled” in “grilled cheese” is more properly “griddled.” (A “griddle” in US English is a flat top “grill,” not a grate or grid iron.)
That’s “cheese toast”. Entirely different thing. In American, any non-meat dish that is “grilled” is pan-fried in oil or butter, but probably butter.
You can but a little pump bottle for spraying the bread with oil, or you can just put a bit of oil on a plate and coat the bread by rubbing it gently over the surface.
Personally, I prefer frying in butter. Margarine I would use only if were really desperate.