Create lots and lots of alliances with top ranked countries. They can aid you in trouble and they make good trading outposts. (But beware of being dragged into one of their wars; if they ask you to help and you say 'no,' you'll lose tons of prestige!) Don't make alliances with small countries........take them over instead. Small countries just get in your way. If you are U.S.A take over Texas before Mexico does..........then take over Mexico.
You shoudn't, however, attempt to form an alliance with a country you plan to take over, as this will cost you Prestige. For example, if you DO plan to take over Mexico eventually, don't form an alliance with them.