Diplomacy is kinda 'broken' or lame.  Only way for them to 'like you' is to have 'strong' military.  There is a list of modifiers relating to your current 'relationship' with each sovereign - on the gov't tab (I think).  It seems that the actual quality/str/yada of your mil units is not as important as the sheer number of units.  So have lots of cheap garrison troops hanging about, use cheapies to 'patrol' the approaches to your domain,etc.  As time resources allow, upgrade troops. Hope helps.