If I want to find out how often I mentioned my flatmates Alberto and Melo in the same entry, I run
jrnl @alberto --tags | grep @melo
And will get something like @melo: 9, meaning there are 9 entries where both @alberto and @melo are tagged. How does this work? First, jrnl @alberto will filter the journal to only entries containing the tag @alberto, and then the --tags option will print out how often each tag occurred in this filtered journal. Finally, we pipe this to grep which will only display the line containing @melo.
You can do things like
jrnl @fixed -starred -n 10 -until "jan 2013" --short
To get a short summary of the 10 most recent, favourited entries before January 1, 2013 that are tagged with @fixed.
On OS X, you can use the fabulous iA Writer <http://www.iawriter.com/mac> to write entries. Configure your .jrnl_conf like this:
"editor": "open -b jp.informationarchitects.WriterForMacOSX -Wn"
What does this do? open -b ... opens a file using the application identified by the bundle identifier (a unique string for every app out there). -Wn tells the application to wait until it’s closed before passing back control, and to use a new instance of the application.