Push on the wall.  This is not a metaphorical encouragement to seek innovative solutions.  Literally, place your hands against a wall and give it a shove.  Now, assuming that you are not working in a cubicle, you have just experienced what physicists call a normal force.  The wall pushed back against your hands with the exact same force you used on the it.

Unless you have superhuman strength... Sorry Superman

One of the prerequisites of a flow experience (as defined by Mihaly Csikszentmihalyi in his seminal work Flow: The Psychology of Optimal Experience) is that the difficulty of the activity roughly match the ability level of the participant.  If it is too easy, they will not become completely absorbed and lose themselves in the activity.  If it is too hard, they will become frustrated and unable to make continual progress.  Balancing between these two extremes is the responsibility of the game designer, but often there is not a single setting that works for every player.

One solution is to allow each player to choose their own difficulty level before starting the game.  Unfortunately, the vast majority of players simply choose the default option in their haste to start playing.  Very few players will admit to needing to play on Easy difficulty, and even good players may be too intimidated to start out on Hard.

I'm tough like week-old bread

Another solution is to dynamically change the difficulty of the game based on an ongoing evaluation of the player’s skill.  But in practice, this automated system usually destroys the intended pacing of the game.  A well-designed game will have periods of less challenge that lead to a more difficult section, building and relieving tension through the gameplay.  A dynamic difficulty system that is too sensitive will add wild fluctuations on top of these natural curves, obscuring the overall effect and flattening out the tension.  One that is tuned to adjust more slowly will often react to the easier segments by ratcheting up the difficulty at the same time that the designer is increasing it for pacing reasons, resulting in an unintentionally large difficulty spike.  Not to mention that the player’s sense of accomplishment will be undercut if they realize the game is “letting them win” by compensating for their low skill levels or “cheating” by getting harder as they get better.

A better solution is to learn from the wall’s normal force and design mechanics that match the player’s skill with reciprocal difficulty.  (Calling this technique normal difficulty seemed confusing.)  In a game with reciprocal difficulty, the more aggressive the player is, the harder the game gets.  But when the player becomes overwhelmed and stops pushing, the game immediately gets easier.  This allows players of all skill levels to be challenged; a good player will play more aggressively until the game gets hard enough, and a less skillful player will proceed at a slower pace until the game gets easy enough.

Examples of reciprocal difficulty mechanics:

• Recharging Health – If a player is too aggressive and tries to fight too many enemies, they will die quickly, but as soon as they withdraw from combat their health recharges and they can proceed more carefully.
• Defensive AI – Enemies that are much more effective when fighting from a defensive position are naturally more difficult when the player is pushing hard against them, but allows a timid player to pick them off slowly without putting pressure on them by chasing them.
• Optional Objectives – Puzzle games that can be solved simply by any player, but have optional collectibles or rewards for completing a puzzle in fewer moves provide more challenge for better players.
• High Scores – Any mechanic that encourages players to play for a faster time or to score more points is better able to satisfy a variety of player skill levels.
• Character Leveling – If a player is having trouble with a difficult section, they can make it easier by earning experience and making their character more powerful.
• Press Your Luck – The player is allowed to periodicaly save their progress or reduce their difficulty level, like going back to town in Diablo or repairing their aircraft in Crimson Skies, but good players will take this option less frequently.