Check out this great tip from nion's blog:
I've often wanted to open up new port forwards, but haven't wanted to shut down my existing session.
If you follow this by # character (and thus type ~#) you get a list of all forwarded connections.
Using ~C you can open an internal ssh shell that enables you to add and remove local/remote port forwardings
Commands: -L[bind_address:]port:host:hostport Request local forward -R[bind_address:]port:host:hostport Request remote forward -KR[bind_address:]port Cancel remote forward
ssh> -L 8080:localhost:8080