For Xcode 3.x, you can use the following:
If you open up a terminal session and enter
defaults write com.apple.Xcode XCCodeSenseFormattingOptions '{ "BlockSeparator" = "\n" ; }'
it will start new blocks of code on a new line. Note that you will have to restart XCode if you have it opened in order for the new defaults to be read and used.
For Xcode 4.x to 6.x (current) I suggest you use Snippet Edit as a handy tool for editing the snippets that define the Xcode autocompletion. At the current time it’s free and works great.