Assigning reserved IP for Amazon EC2 instance

I want to assign a reserved IP for an instance on EC2 so that I can use it in my configuration files. I associated an elastic IP with that instance but it changed when I stopped the instance. I have to setup application which depends on IP of instance, if IP changes, my application will fail to work. Please suggest a way so that the IP remains associated with instance permanently.

Deploy inside of a VPC. When you do this, you get many additional benefits, one of which are EIP mappings that persist through reboots.

VPC usage is free, they're quite simple to set up, so there's no good reason to not use them.