Two techniques I've used --
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One year, I made myself do a sketch a day of something that reflected an event/feel of the day. They were sometimes cartoons of funny things that happened, sketches of mundane meetings, or seasonal drawings (for example, the first day in spring that the frogs started chirping, I drew a frog in the pond.) It ended up being a cool sketch diary of my life that year and helped me pay better attention to my day because I eventually started watching for things to draw. (It also made frustrating events more bearable because I'd be thinking, "How can I make a cartoon out of this?")
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Other years, I've chosen a category for the day -- animals, furniture, landscapes, portraits, etc. -- and then googled the category and made myself draw the first thing on the google image list. This method has the advantage that you can just pick several categories that you keep repeating for awhile.
I will add that whenever I've really forced myself to do a sketch a day, I am reminded of how much my drawing improves as a result. It's not always an easy discipline but well worth it.