H is for How.
"How?" is the most important question to ask yourself when you're adding tasks to your to-do list.
Just as you're about to stick "clear out the shed" onto your list, stop and ask yourself "How?" As it stands that's a pretty big and woolly job. Would you know just where to start with it? For me it sounds like one of those nasty, complicated jobs that will take an age and so I'd end up putting it off and putting it off.
By asking "How?" you start to break it down and make some of the decisions about how it will be done. Maybe one of those new tasks would be "Throw away the old gardening supplies". Query that with "How?" too. That could yield "Fill a black sack with all the broken plant pots and seeds that are past their best-before date".
That may be enough, or you could take it further - down to checking that you've got enough black sacks. You never know, the "clear out the shed" project might start with a trip to the shops to buy more bin bags.
Repeatedly asking yourself "How?" forces you to do the thinking about the task now. It stops you from using your to-do list to delay making a decision. Instead, your task list becomes just that - a list of tasks that you've already decided to do, and how to do, so you can work through them much more quickly.