I think you should un-ask the question of “is it a placebo or does it actually work”.

The placebo effect is when a treatment has an effect primarily due to the psychodynamic effects of administering the treatment. and the expectations that the subject has surrounding the treatment.

When you meditate/write a journal/etc you are consciously using psychodynamic effects to benefit yourself.

...on second thought, I suppose the question is valid if you are referring to positive effects due to faith in the technique (as opposed to the technique being effective). For example, in meditation experiments the control group is sometimes given an alternative set of instructions and they are told that it is a traditional form of meditation, when it really isn't.