Do you link similar threads with similar subjects?
Often there are two email threads that have separate tasks in them, but that both relate to a very similar topic. In this case, it can be useful to give them the same subject, with a number afterwards to differentiate them.
SSW Rules - Add 'please update this rule' button #1
SSW Rules - Add 'please update this rule' button #2
Read Do you realize the importance of a good email Subject? for more on good email subjects.