Hot/cold packs are great at alleviating pain and reducing down time due to injury. It is well known that heat will relax muscles and cold will reduce inflammation, no secret there. Most people don't want to spend their hard earned money on ice packs and heat pads, its just not worth the money when you have the raw materials in your house.
To make a cold pack for those inflammed muscles simply pour a liter of water into a sealable zip-lock bag. Add a quarter cup (1/4 cup) of isopropyl alcohol or half cup (1/2 cup) of salt, both will do the trick. This will turn the ice into a sludge that you can throw in the freezer and reuse as it will not become a block of ice in the freezer. Because it is not a hard block it will form fit to the body part you are icing. The alcohol/salt will also lower the melting point of the ice allowing the ice pack to stay cooler longer. It is wise to put the zip-lock bag in another zip-lock bag to avoid any leaking.
To make a hot pack simply take an old tube sock and dump as much rice as will fit and still allow you to tie a knot at the end. Simply throw this into the microwave for 45-60 seconds and you have a hot pad ready to alleviate any aches and pains from sore to tight muscles. Rice is very good at retaining heat so be careful not to over heat in the microwave. This hot pad can be reused over and over without replacing the rice.