Sooooo.... I stole this from another website, but its such a great summary.
Ice or Heat for Herniated Discs / Which to Use?Ice is an anti-inflammatory agent which will reduce swelling and temporarily retard circulation when applied. Ice is also an anesthetic agent and will reduce pain in muscular tissues. However, ice will also enact a reflex response as the body reacts to the cold by expanding blood flow to the affected area.
Heat increases cellular activity and circulation. Heat loosens tight muscles and helps in the removal of cellular waste products, due to increased blood flow.
For most injuries, ice is recommended for the first 24 to 48 hours and heat is recommended from then on. Ice should be left on for 15 to 20 minutes at a time and the area should be allowed to warm for at least 30 minutes between applications. Heat can be applied liberally and is especially effective when wet heat is utilized.