I'm not sure if this is common knowledge by now, but I had o dig through a lot of threads (on different forums) until I found this info, so I decided to post it here, hopefully it will help some people.
So, after some time trying to get summoned by a friend near a fog door (restarting, replacing signs and all that), I decided to do some proper search and I came across this, and it really works:
First you'll need to be on the start menu, then you go to your friend's gamertag page(or whatever it's called) on gfwl, and there you'll be able to select "join game in session".
What this does is put you both on the same session, then you select your character, and if you meet the normal summon requirements, your friend's sign should always appear now! (Please note it will still take some seconds for it to appear on your screen, so constantly replacing signs is not advised)
This made me really happy! Yes, it's not intuitive at all, but atleast there's a way to do it, and save large amounts of frustration and time. We managed to do 3 bosses (including 3 runs on one of them) without any issue! A couple of times I even saw his ghost running and stopping by my summoning sign. Really cool stuff.
Hope this helps someone.
Edit: I'd love to credit the author of the thread, but I'm at work and the those forums are blocked here. I'll add it when I'm home and have time to do it
Edit2: Here's the thread: http://preparetodie.com/forums/viewtopic.php?f=18&t=4546
Last edited by Purity on Mon Sep 24, 2012 8:08 pm; edited 1 time in total