Use “since then” to talk about an action that started at a specified point in the past and is still happening today. As in:
“I met Julie when I was in university. We have kept in touch since then.”
This means that I am still in touch with Julie.
I should not say:
“I met Julie when I was in university. We have kept in touch until now.”
The problem is that “until now” implies that I just stopped keeping in touch with Julie.
I can’t find a good reference in a grammar book, but to my ear “until now” always means that the action has just stopped. As in:
“Until now, I have gotten good grades.”
This means that I just stopped getting good grades.
Basically, “until now” implies that a change has happened at the present time.
For more information about how to use “until now” check out this blog post.