The players for each league's team are selected through the following process:
Fan voting (8 NL players; 9 AL players): Baseball fans vote on the starting position players for the All-Star Game, with ballots distributed at Major League Baseball games before mid-season and, more recently, on the Internet. In games with the designated hitter, the American League DH is also selected in this manner (and the National League DH is selected by the manager). Fan voting has been recently criticized because most of the starting players can come from teams that have large fan bases, such as the New York Yankees and the Boston Red Sox.[4]
Player voting (16 players): Eight pitchers (five starters and three relievers) and one back-up player for each position are elected by the players, coaches, and managers.[5] If the top vote-getter at a position has also been selected via fan voting, the second-place finisher in this category is selected.
Manager selection (9 NL players; 8 AL players): The manager of each league's All-Star team – in consultation with the other managers in his league and the Commissioner's Office – will fill his team's roster up to 33 players. The NL manager will also select his team's designated hitter.[3] At this point, it is ensured that every team is represented by at least one player.
Final vote (1 player): After the list of 33 players for each league is announced, fans vote (on the Internet) for one additional player, chosen from a list of 5 players that is compiled by the manager of each league's team and the Commissioner's Office.
Replacements: After the roster is selected, the All-Star manager and the Commissioner's Office will replace players who are injured, decline to participate, and pitchers who started on the Sunday before the game.
http://en.wikipedia.org/wiki/Major_League_Baseball_All-Star_Game