Try this first:
http://developer.apple.com/library/ios/#documentation/ToolsLanguages/Conceptual/YourFirstAppStoreSubmission/ProvisionYourDevicesforDevelopment/ProvisionYourDevicesforDevelopment.html
Earlier in the year I registered in the apple developer program. I created an iOS 'app' using Xcode on my MacBook and uploaded it to my iPod to test. Then I didn't use my Mac for six months.
Recently I noticed my app on the iPod stopped working, so today I turned on my Mac and attempted to upload it again. Xcode failed to build and upload the app as my provisioning profile had expired. I wasn't expecting this as my developer account should be ok for a year.
As a complete novice in the world of iOS application development I didn't really know where to start fixing the issue (I really can't remember how I got a provisioning profile in the first place). I tried looking in the Xcode menus but couldn't find anything. Then I thought I may need to be upgrading to the iOS 5 as I had had several e-mails about iOS 5 beta. This got me to log into my developer account where I was immediately asked to agree to new usage terms.
I was now at http://developer.apple.com
I clicked on the 'iOS Dev Center' link as it looked promising and then noticed a link on the right; 'iOS Provisioning Portal'. This proved to be the start of the solution.
I clicked on the link 'Get your application on an iOS with the Development Provisioning Assistant' and this led me through creating a 'provisioning profile', a file I was asked to drag and drop onto the Xcode Organizer window (after first making sure the 'Provisioning Profiles' tab was selected on the left). The file was called myprofile.mobileprovision as was about 14KB in size.
After this was done I was able to successfully build my app and upload it to my iPod.
