I see this is a very old post, but in case anyone comes here looking for this I'll add a response on here.

I think there are several things that are missing in the above code. For one thing, you have to get the "working copy" of the work item, not just the "new state". So that would look like:

IWorkItem newState = (IWorkItem) saveParameter.getNewState();
IWorkItemServer wis = getService(IWorkItemServer.class);
IWorkItem WI_WorkingCopy = (IWorkItem) fWorkItemServer.getAuditableCommon().resolveAuditable(newState.FULL_PROFILE, null).getWorkingCopy();

Note that the above instance of IWorkItemServer necessitates adding "com.ibm.team.workitem.service.IWorkItemServer" to the prerequisites of the plugin.xml in the Manifest file. There are posts on how to do this elsewhere in these forums.

Then once you have the working copy, you modify that working copy in whatever fashion you choose. After making your modifications to the working copy, you have to save it for those changes to be committed to the database. That command looks something like this:

service.saveWorkItem3(WI_WorkingCopy, null, null, additionalParams);

Note that the "additionalParams" argument should contain a string that you can check for at the beginning of your follow-up action to prevent the follow-up action from being triggered by itself recursively. To set up your "additionalParams" variable, it should look something like this:

Set<String> additionalParams = new HashSet<String>();
additionalParams.add("EnsureThisStringIsUniqueSoYouCanCheckItLater");

At the beginning of your follow-up action, you should check for this string. That code would look something like this:

if(saveParameter.getAdditionalSaveParameters().contains("EnsureThisStringIsUniqueSoYouCanCheckItLater")) return;