Does that mean you're creating your reference list manually? If so, DON'T.
Most standard bibliographic packages (Endnote, Reference Manager, Mendeley, Zotero) will abbreviate the journal names as required by the specified journal assuming they have the right style settings for that journal (you may have to download them)
If you're not already using one of these, start now. Really, there's no good reason to want to do these things manually all of the time.