I would like to share what I tried in this similar line.
I conducted a team building session for a 30 member team. This session had a clear set of defined objectives. We had activities around each development area (which i identified prior to the program) and after each activity, i asked the team to come up with action plans for that particular development area, then I formed a focus group (4 members) giving responsibility to this people to see that this action points (top 4 in the list) are implemented and practiced across the team. And twice in every week i had a one hour session with the focus group to check on the progress and observations and support required for the team.
I did tried to do a mystery Audit to find out and document the effective of the program, which i couldn’t complete 100%, however this did yield benefit as I received good responses from the management on the change in the team performance. (Especially on the identified development area).