Sometimes I decide I want to increase the salience of an area of focus: that is, I want
to get in the habit of spontaneously thinking about it more often.

I put reminders in places where I'll see them repeatedly. I might leave objects
relating to the area lying around where I'll run into them, write the name of
the area in big letters on my whiteboard, etc.

I don't expect myself to necessarily do anything specific about the area when I see
the reminders, but after seeing such reminders over and over again over a number
of days, the area becomes more active in my subconscious mind and I'll be more
likely to think up projects and actions related to it, or to decide to actually do such
actions if they're already on my lists.

GTD isn't just about getting stuff out of your mind. It's also about getting stuff
into your mind (e.g. during weekly review). By clearing away other stuff, you make
more room for the things you do want to spend more time thinking about.