Also, having a surplus of diplomats, because I have found that if you keep asking eventually you will probably be successfull, all other factors being the same.
Also, if you are playing one of the non-european nations near the Indian subcontinent or South-East Asia, especially in later stages of the game, watch for a colony to revolt away and become a one or two province country. Bengal from England is a good example. Make haste to make war to sack the capital or annex, because that new country will have all the map knowedge of its former European colonizer/oppressor.
Ron