To use elements from existing documents in a new or different document, the fastest and most accurate way to do this that I have found is by creating and using Libraries. You can share aspects of your design so quickly and effectively, and you don’t even need a library card!
To use an element from your Library in your document you can Copy Cmd + C and Paste Cmd+P it in. Or even better use the Paste-In-Place Cmd+Opt+Shift+V command to keep the exact location as the original which is darn handy.
To create a New Library open the File > New > New Library. It will ask you where to save the Library file and once you choose a place it will show up as a floating panel. Then you can just select and drag elements into the Library panel – simply double click on the element to name it and create its description. You can email Libraries and share the elements, with their exact relative positions, to maintain consistency. This is a very helpful feature if more than one person develops a document.
Oh yeah, if you use a Windows OS, use these instead: