I think
@mcogilvie hit the nail on the head: experiment and find the trigger that works best for you. I do it multiple ways.
Sometimes the next action triggered by the completion of something I've delegated is obvious. If I've sent a proposal to my boss for approval, upon receiving that approval I send it to the customer. I don't need a trigger; the next action is obvious.
Sometimes, like
@mcogilvie suggests, I'll append it parenthetically to the waiting for item in my list.
Sometimes I include the information in project support. Once the thing I've delegated has been completed, it's natural for me to look there. In my case, I use software called Nirvana which allows me to add notes to a project.
As an aside,
@Longstreet is correct that the default for projects in Nirvana is sequential. I would guess that works if most of your projects are sequential (i.e. only next action to be done at a time) and can be planned out in their entirety ahead of time. Most of my projects are not like this, so as is suggested in the GTD guide to Nirvana offered by the David Allen Company I've set the default for projects in Nirvana to parallel. That works best for me.
I'm not going to evangelize Nirvana -- it is only one of many good tools available -- but I do think it's a decent option and as
@Longstreet points out it does offer some automation that can solve the issue brought up by the OP, at least for some people in certain circumstances.