Nonweapon Proficiencies (DMG)

Nonweapon proficiencies are optional, but, if chosen, can be very useful. If you are uncertain whether to use these proficiencies, the following points should make the decision easier:
Nonweapon proficiencies help determine the success of character actions beyond what is defined by the basic abilities of the character races and classes. They provide a useful gauge when a character tries to build a boat or behave properly at court. This frees the DM to think about more important parts of the story instead of little, perhaps even insignificant, details.
Not everyone agrees with this! Some DMs prefer to handle by themselves all the situations covered by proficiencies. This requires a quick wit and good memory. In return, the DM is freed from the restraints of rules. He can create the scene he wants without worrying whether it breaks the rules. But tread softly here—this is not an easy way to judge a game! Try this only if you are experienced at DMing or are a spontaneous and entertaining storyteller.
Nonweapon proficiencies give a player character more depth. Used cleverly, they tell the player more about the personality and background of his character and give him more tools to work with. Applied judiciously and thoughtfully, nonweapon proficiencies vastly increase a character's role-playing potential.
Beware, however, because nonweapon proficiencies can have exactly the opposite effect. They can become a crutch for players who are unwilling to role-play, an excuse not to develop a character's personality or history. Some players decide that proficiencies define everything the character knows; they make no effort to develop anything else.
Avoid this by encouraging players to dig deeper and explore the possibilities in their characters. Ask a player to explain why his character has specific proficiencies. What did that character do before becoming an adventurer? Questions like this stimulate players to delve into their characters' personalities and backgrounds. Make a note of the player's reasons and then you can use them during play.
Nonweapon proficiencies can be used to define the campaign and create atmosphere. The proficiency lists can be tailored to match specific regions or historical periods, or to define the differences between nationalities.
If the characters' home base is a fishing village, the lists can be altered to allow all characters to learn swimming, sailing, fishing, and navigation at the same cost (in proficiency slots). These are common skills among seafaring people.
At the same time, dwarves, who come to this town from the nearby mountains, must devote extra slots to learn these proficiencies. A youth spent in dry, solid tunnels hasn't prepared them for a life at sea. Instead, they can learn mining, gemcutting, and other stonework skills cheaply.
The proficiency lists in the Player's Handbook are only a beginning. Your campaign will develop a much more interesting flavor if separate lists are tailored to different regions.
This still leaves the problem of min/maxing. Players are encouraged to make intelligent and sensible choices for their characters, but not at the expense of role-playing. If tailored lists are in use, encourage players to list the proficiencies they want without getting to see the lists of proficiencies. Then collect the lists and figure out which proficiencies the characters can get (some may be unavailable and others too expensive).
Players will still request the proficiencies they think are most advantageous, but at least the selections are drawn partially from the players' imaginations instead of a list of numbers.
Finally, proficiencies are only as useful as the DM makes them. Once a decision is made to use proficiencies in the campaign, the DM must strive to create situations where they are useful. Always remember to design encounters, traps, and scenes where proficiencies have a practical application to the problem at hand. Otherwise, players are going to write off proficiencies as a waste of time and miss out on a wonderful chance to expand their characters.
Ultimately, proficiencies add much richness, detail, and role-playing to a campaign at only a small cost in increased complexity. The DM has to remember a few more rules and the players have to make a few more choices when creating their characters. But in return, the game is bigger, better, and more fun.