Basically, the best way to do it is to either slow down Western Europe's technological and political progression by a century or two (fairly difficult, IMHO), or to have an ERE that retains its dominance in Western Asia by the time of the emergence of powerful European states, notably Hungary and Norman Sicily. Without these factors, the Empire with a post-700 POD will always be facing an uphill struggle against aggressors on two fronts.
Alternatively, just butterfly the events of the first half of the seventh century, and the Roman Empire can remain a Mediterranean superpower.