Lets break down the Military Discount for all of the major phone companies. Most are offering 15% off part of the bill. There are additional military benefits as well, like getting activation fees and such waived in addition.
AT&T - 15% OFF
AT&T offers 15% off the main plan. If you have additional people on the plan, the discount is not applied to the cost of additional users. So this isn't as good as it seems when you have 6 people on your plan.
Verizon - 15% OFF
Verizon offers 15% off the monthly bill for military members. They also offer options to terminate your agreement and waive any fees when you are deployed
Sprint -10% OFF
Sprint gives military members 10% off.
Ended their military discount in 2014 :( but now have a new military discount that started in 2018
The easiest way to enjoy these discounts is just to go to a local store and show them a form of Military/Veterans ID, DD form 214, Military Orders, or a Paystub. If you cant visit the store, you can always contact Customer Support and Email/Fax a copy of your Identification. Many of the companies allow you to apply online. I have provided the links above.
Most all of them waive the activation fee or any other associated fees also. For example I recently upgraded to the Iphone 5C and I called customer service and told them I was Military. They gladly reimbursed the $35 upgrade fee. Many companies will do it if you have been a customer for over 3 years.
One thing that bothers me however is that these companies apply the military discount only to the Primary Account Holder. This is a bummer because my brother and I are both on the same account. We are both in the Military, but only my portion of the bill gets the discount. I wish they would change this so that if there were multiple account holders in the military, they could show their military ID and get the discount also or make it so that if they primary account holder was military the whole family or bill would get the 15% discount.
In addition to wireless military discounts, Apple offers discounts on many of its products. You can check them out here