Whatever you are working on should be easy but not boring. Interesting but not frustrating.
The best progress takes place in the sweet spot where things are just challenging enough to be engaging. If you're a tennis player, you'll have the most fun with someone playing close to your level of ability. You don't want to play with a total novice, but on the other hand it's no fun when every serve is so strong that it zooms by before you even know it happened.
There is an art to choosing just the right level of difficulty. Video games are very good at continually calibrating themselves to your ability at any given moment, which is what makes them so addictive.
Whatever skill you want to learn, make a game out of making it a game. That is, figure out how to make it as engaging as possible. As Mary Poppins said, "In every job that must be done there is an element of fun/you find the fun, and snap! The job's a game."
Start with the right material. At the bookstore or library, you put down the book that doesn't grab you right away because it's too dense, but you might also skip over something that looks too trashy or fomulaic. Don't feel guilty about rejecting Ulysses - maybe it will be just the right thing in the future.
When you've got something you can sink your teeth into, you know. Get good at finding that feeling, whether you're a pianist working through classical repertoire, a Spanish student looking for instructional material, or a fencing enthusiast seeking an opponent. Insert Goldilocks reference here.
Make your progress visible. We all rubbed our fingers raw trying to snap our rooms clean with magic, so that is eliminated as a possibility (unless you got it to work for you, in which case please let me know!). But even without magic, many of us don't mind cleaning when we can see the results with every swipe of the rag or push of the vacuum cleaner. Make your progress visible and you will find yourself conditioned to return to a given task.
To get a better sense of your evolution you might take before-and-after pictures, videos, or audio recordings. You could also create a chart or graph of your work over time.
Be a statistics nerd. The drama of an apparently dull moment of baseball can be heightened when the announcer shares a stat like, "He has a .129 batting average so far this season, but a .401 career batting average when facing this pitcher." All of a sudden we're invested in the outcome of this slumping second-stringer's at-bat.
You can geek out on statistics in order to set goals and motivate yourself to grow. In running, a sport where it's just you and the clock, you will find more depth and nuance when you can design workouts that have a specific target in mind based on past performance and future goals. You would play with various combinations of speed and distance (tempo runs, interval training, long runs, negative splits, etc.) in order to optimize your performance for a specific race.
As a musician, you might use a metrononome to track the tempo at which you can play a given phrase comfortably. You could also document how much of your piece you learn each day. I like to track the date a student begins a piece and the date that piece is mastered. As the student breaks through a plateau, we can compare two equivalent pieces and point out that one took two months to master while the newer one took just two weeks.
Avoid unnecessary repetition. If you understand a math concept, you should do a few problems to solidify your understanding today, a few more tomorrow, and a couple at the end of the week. A page of twenty-five problems is busy work if it too easy, and impossible if it you can't even do the first one.
If you don't understand the concept, you'll need to break it down into smaller pieces, look at it from a different perspective, or rebuild the foundation by reviewing previous material. Repetition won't help.
"What's your practice strategy?" I ask. "I'm going to play it over and over again until I get it right!" my student says brightly. No! Play it correctly the first time, and you'll only need to play it a few times to make it solid. If you can't play it correctly the first time, it's too much. Break it down, take it slower, or both.
Use rewards. My students love getting stickers. Not only do they enjoy picking out just the right one (there is an inverse relationship between the age of the student and the amount of time it takes to choose a sticker), they enjoy the closure that it represents ("You have fully mastered this piece!").
Even as an adult, you can reward yourself for the acquisition of a skill or the completion of a goal. Even better, give yourself positive reinforcement for the little steps along the way. Karen Pryor, in her excellent book Don't Shoot the Dog!, tells of using tiny bits of chocolate to get herself out the door and on the bus to an evening class.
While the opportunity to perform onstage might be seen by many musicians as a punishment rather than a reward, applause is certainly a nice way to be acknowledged for all your hard work learning a piece of music. In the meantime, dessert or some down time (TV, a novel, a game) can be a great way to reward yourself (or your child) for a focused practice session. Over time, you will condition yourself to look forward to practice because you'll associate it with the pleasure at the end!
There are many other ways to enliven the process of learning something new. When learning a new skill, apply strategies and techniques from something you're already good at. The more you can make it a game, the more you'll want to play!