This topic is as old as the hills and has been explored by many authors. I have personally been asked the question thousands of times and have likely have given many variations of answers over the past 10 years as a therapist.
There are well known and well defined rules to follow and of course there are exceptions to the rules.
Most people have done a first aid course in their life at some point. Also, you have learned some basics in health class (credit to Mr. Randall, G.C.Rowe Jr High, 1990; Mr.Carberry, Herdman Collegiate, 1994). What we all should remember is the acronym RICE. Essentially, in case of acute orthopedic injury you should Rest, Ice, Compress, and Elevate. That rule has been studied for different types of injuries and in terms of individual parts and in combination with other treatments such as anti-inflammatory medication.
My personal recommendations are as follows. Use ice in the first 24 to 48 hours following all acute injuries, especially those involving visible swelling or non-relenting ache or pain. I will guarantee you that your injury will be lessened in duration if you heed this simple instruction. Place a damp towel between your skin and a mouldable ice pack. Apply for 12-15 minutes as many times as you can daily. Do let the skin warm between sessions. Lessen the time for areas directly over bony prominences or if you are elderly with thinner skin. Apply light compression as well – a tensor wrap works very well.
After the acute stage you may continue to apply ice regularly if you are aggravating condition by just general movements. If pain persists, and especially night pain, I advise continuing with the ice as long as it takes to see the pain lessen. You should consult one of our Physiotherapists if pain or swelling persists beyond the first 2-4 days. Our new treatment procedure will reduce swelling immediately.
One exception is if you are experiencing muscle spasm more than inflammation. In cases of back and neck pain which has acute muscle spasm, heat application is beneficial. However, the safest thing to do is to ice first. If you apply heat in the first stage of injury you may worsen inflammation, swelling and bleeding.

Ashley Buckle PT MCPA