There are several factors that can help an empire achieve a higher level of unification. Most empires that have become strong and long-lasting shared some or all of these characteristics.
An important characteristic is strong trade or a strong economy. This serves several purposes. It allows the people to be in contact with other communities. It also keeps the population happy, which means that they are more likely to tolerate their ruler. Another factor would be cultural and religious tolerance. In the case of Rome, this was an important factor, as part of the success and stability of the empire could be attributed to their religious and cultural tolerance. Finally, a third factor would be a strong government or leader. This is an important characteristic for the leader of an empire, as it would make him more admired, as well as more feared, within his territory, decreasing the chance of rebellion.
