This MacGyver tip comes from a Lifehacker reader:

“My wife pulled her sciatic nerve last week. An ice pack was recommended but one of our friends gave us a great tip. Instead of an ice pack, partially fill a strong zip lock bag with Dawn dishwashing detergent and freeze it. (I don't know if other brands would work just as well.) Just to be safe, we double-bagged it! The detergent stays cold much longer and it can be re-frozen over and over. It also molds to your back better than ice.”

What a great idea!