There are probably as many ways to go about character generation (chargen) as there are referees. I've always been a fan of "3d6 in order" chargen for several reasons. Chiefest among those reasons is the fact it's simple and fast, my main criteria for rulings in my campaign. I like to keep the action moving.
Almost as importantly, however, is my belief 3d6 in order removes the emphasis from generating the perfect character and gets the player to rolling dice. Ability scores in original edition Dungeons & Dragons are simply not as important as in later versions of the game, it is the player's skill that makes the difference.
Still, there is often a need for a certain type of character in the game. If the party consists of four magic-users, a new player rolling up a fifth one might not be the best addition to the party. The dice are tools, not dictators and though it is fun to let random chance have a hand in all aspects of the campaign sometimes a bit a self-determination is in order. My solution? I offer two ways to alter rolled ability scores in my game.
The first method is "point sell" system (as I call it) in the rulebooks. Each class can "sell" points in 2 different ability scores to add to their prime requisite. I keep the same restriction as in the books, that is, no ability score can be reduced below 9. This is a good method if the dice have rolled a character similar to what the player already wants, but wants to "beef up" the prime requisite a bit.
The second method is much simpler but more useful for when the dice roll a complete miss for a certain needed (or desired) class. In this case I allow the player to swap any single ability score with the prime requisite of the desired class. For example, if the player rolled a perfect Cleric character with a 17 Wisdom and 5 Strength but he really wanted to player a Fighter? Under Method II he could swap the WIS and STR scores and wind up with a pretty strong Fighter who may be a bit lacking in common sense.
Naturally, a player may choose one or the other method as desired ...but not both!