To take a stab at what I know of that topic:

• offer help, but don't inflict help that isn't requested
• verify that the helpee is "serious" about using your help: help can't be for free
• an intervention is also a a test of a hypothesis: update on the results
• as a corollary, effective help requires forming a theory or model of the situation
• the best way to get entangled with the situation is to listen to the "helpee"
• listening requires an open mind (i.e. often changing your mind)
• the helpee's situation is a system, with many entangled components, which can include other people
• your help and intentions in helping can become part of that system, for good or ill
• your help, intentions, approach and results should always be a legitimate topic of discussion with the helpee
• you should always be clear about why you're helping
• because of that, it's often a good idea to have someone in turn helping you help others

That's from my general approach to consulting, i.e. helping people, or more precisely "influencing people at their request". It's not specific to grief or depression counseling, and thus should perhaps be taken with a grain of salt.