Don't know if this would be much help since I have be having some on and off luck with the techniques Gackt mentioned.
Anyways, I've been playing with numerous friends from both the US and UK, and as mentioned before sometimes Gackts methods work, but my theory is that once you place your summon symbol down and leave it down, it cycles the servers. (Kind of reminds me of Gears 2, constantly searching for match making)
I mainly tried this with my friend from the UK, he left his summon sign up without taking it off and putting it back down, and eventually I found it (given enough patience) and I know we both weren't on the same server.
Also another helpful hint is that the 1st time you connect is a pain, 2nd time is a little less longer, then after that just about every time we place down our summon signs the other player immediately sees them.
Of course this is all my theory, but obviously the best would be both being on the same server.