Update: This information is accurate but, since this Reminder's first posting, the AWS Console now offers Elastic-IP management. It might be easier to manage IPs from there I guess.
To do this, you must own an Elastic IP. If you don't, just create one in the AWS Management Console. It's free.
Type this to view the Elastic IPs you own:
Then type this to view the instances you own:
Then you'd do something like this:
ec2-associate-address -i i-b32b857 18.104.22.168
But use your own instance id and your own elastic IP rather than the ones I've used in this example.