In my blog post The Long Dark Summertime of the Soul, I gave a list of a half-dozen refresher courses that were running for the summer of 2011. Many, such as the course in Lisbon, are run on a regular basis (i.e. annually or biannually), so they are worth checking out.
The University of La Laguna sometimes also allows professional (i.e. trained) conference interpreters wishing to brush up on their technique to audit their classes for a couple of weeks in exchange for working with their students. This is particularly useful if you have Spanish in your combination.
One thing to keep in mind: refresher courses aimed at working interpreters tend to be run in low season, for obvious reasons, which means mid-July to early September, and early January.
answered 18 Oct '11, 23:53