I believe there are two ways to challenge yourself:
You can play increasingly difficult music OR...
You can play simple music increasingly well.
All too often, young players devote too much of their practice time conquering difficult material and too little time and attention really polishing easier music.
We all lead busy lives filled with deadlines and competing demands. Each day something comes up to take away practice time. When pressed for time, there is always the temptation to skip the easy stuff and get right down to crunching the hard licks. For most players this is a fundamental mistake.
Look at it this way:
Just as we need to eat a balanced diet, we need to have balanced practice sessions.
While long tones are good for developing tone quality, I prefer slowly moving patterns and simple, lyrical melodies. It is crucial that everything you play have a sense of musical direction. Static patterns such as long tones, while they can played musically, fall easily into the trap of mechanical, static playing.
Click here for a collection of 12 simple tunes you can download and print out!