I'm often waiting for things that are outside of my control that I want to remember to follow up on at some point in the future. I use RTM in different ways to track these items and ensure that I follow up. In my case, I prefer to follow up weekly but YMMV. I am careful to have only 1 reminder for any single thing, but these are in different places and I prefer to keep them separate - emails, physical items, and RTM tasks. I also have separate reminders for each of these 3 types because I can do them at different times and/or places.

For emails, I have a Waiting folder in my Gmail account that I breeze through. I have a reoccurring RTM task to remind me to review that folder each week. Items in this Waiting folder include emails I sent to clients where I need a response and items I've ordered but not yet received.

Some triggers that help me track items I am waiting on are physical in nature. Examples include a UPS receipt for a package I mailed that I need to make sure has arrived, gift cards that I want to use at some point in the future, and a quote from a painter that will get us on their summer schedule. I have a reoccurring RTM task to remind me to look through the items in this folder each week on Monday.

For RTM tasks that I need to wait on, I have a "waiting" tag that is used to populate a "Waiting" Smart List. A reoccurring RTM task reminds me to review this list and I choose to do this every Monday morning as I start my work day. Some examples of things that make it on this list:
- a reminder to buy a new outdoor table set (waiting for the husband to agree on the timing of this)
- an annual task to put out the hummingbird feeder - it will sit here until I spot the first hummer; I record the annual arrival date in a Note in the task
- client-related tasks for my small business that cannot yet be done for (waiting for a physical item to be dropped off, requires a meeting that has not yet been scheduled, etc).