I've been struggling to win at emporer and above but finally realized that the key (at least for me) is to be aggressive early. Conquer a few city states and the closest civ within the first 50-100 turns (I play epic). The reason is you need lots of cities fast, and you can't settle them fast enough to keep pace with the AI, so you just take them. Once you have your wide city base then you shift focus to your win scenario. This last week I finally beat Domination, Science (about 15 cities), and Religious (about 20 cities) on emporer. Still haven't figured out culture victory yet. Key is, you can't be friendly at these levels and you have to be aggressive!