The following is from http://www.onegoodthingbyjillee.com/2012/04/almost-effortless-way-to-clean-your.html
Bought a box of cheap dryer sheets at the dollar store (I think they literally cost me one dollar), put the oven racks in the bathtub, threw in about 6 or 8 dryer sheets, and covered them with hot water. For good measure I decided to pour in about half a cup of Dawn dishwashing liquid as well. I figured it couldn’t hurt. :-) I let it all soak for about 18 hours. (Although I’m sure it didn’t need to go that long.) Then with the racks still in the tub I “scrubbed” them with the dryer sheets and amazingly almost all the “gunk” came right off! For the extra tough stuff I used a scrub brush, and between the two, 99% of all the “gunk” was gone!