I have recently seen someone say to use salt and a cut potato...also someone else made a paste of vinegar and baking soda, I think...I like Barkeeper's Friend - its in a canister like other powdered cleansers...
If you use wax paper then there is no mess to clean up after, and you dont have to use any cooking sprays of any sort either!
I use a baking soda and vinegar and water paste. Spread baking soda an water paste over the baking sheet use a spray bottle filled with vinegar, spritz the area and let stand for about 15min. then scrub. After cleaning it to your satisfaction,use parchment paper for future baking.
I would do it as @Colleen suggested, adding a cut lemon to scrub the mixture.
put the baking sheets in a large plastic garbage bag and pour some ammonia in with them. Even soak some paper towels with ammonia and lay them on the sheets. In any case enclose the tray(s) in the plastic bag and allow the fumes to do their job. The ammonia will soften and remove the baked on crust. Then take them outside and use a garden hose on them to rinse them off. A good quality oven cleaner will also work. As long as they are not aluminum but steel.
I put my baking sheets upside down in my oven and set the oven to clean, two things at once and they turn out beautifully! All the residue is off!
Although Anita's solution sounds great for this, I'll add one that is multi purpose and doesn't give off funes.
I inherited some nice old roasting pans that had that brown baked on film on them. I tried a straight razor in a good handle. That brown film (and any residue) can even scrape right off in sheets. After one or two scratches, you get the feel for the right angle and probably won't scratch anything again. So easy I went overboard one day and cleaned all I had, incl the bottoms of pots.
Don't buy the cheap $2 handle for the razor - get the good $4-5 one that has rubber on it to cushion your hand. Since then, I use this tool on almost anything flat and solid - metals, kitchen laminate, stove tops or coil burners, ceramic, glass and even (cold) candle wax off my antique wood table.
Tip, keep turning it over in your hands to keep the razor from getting a bur, check your angle and change to a new blade for more delicate surfaces. It takes a try or 2 to get the hang of not scratching and then you're set. It is, hands down, now my favorite cleaning tool in the kitchen.
Best example: A friend inherited a good glass top stove. Knowing they were replacing, the previous owners let a major crust (as much as 1/4" high) build up around each "burner" - had to see to believe. My friend asked what he should buy to soak and scrub it off. I envisioned months of soaking, scrubbing... Afraid that he would freak when I showed up with a razor, I quickly scraped two circles clean while he was out of the room. Totally clean and not a scratch. He thought it looked like fun and did the others himself - still no scratch. Just make sure the blade doesn't get burred or bent as you go.
A magic eraser!
YES! Iove them so much, I'm addicted! I actually got black mold out of an area under the furniture. I did add a little cleaner. But I was careful not to tear up the eraser. I fold it in half for jobs like this, then it's thicker. I just use the eraser in all four directions of the carpet fibers on the spot & it worked! I also use them on fabric like clothing. Again I use it in all 4 directions of the fabric. Some things on clothes come out, a few dont.,doesn't seem to work well on red stains. Takes nicotine stains off of anything. Do easy without cleaner. I got it bad! Mr. Clean is my man and looking good!
Magic eraser. Use on almost ever thing including fabric and carpet spots. Love them so much!