Once an NHL player has played in 180 NHL games, or has played three seasons since signing his first NHL contract, he must first clear waivers before he is assigned to a minor league affiliate.
After a player has been placed on waivers the other 29 NHL teams have 48 hours to put in a claim for that player. If only one team makes a claim for the player, then he will be transferred to the claiming team. If more than one team makes a claim, then the player will be transferred to the team having the lowest percentage of possible points in league standings at the time of the request for waivers. If waivers are requested outside of the playing season, or before November 1st, then the player shall be transferred to the team with the lowest points in the preceding season. If no team places a claim the player can be sent to a minor league affiliate.
When a player clears waivers and is sent down then is called up again, he would not have to clear waivers to be sent down again unless he's played 10 games or been "up" for 30 days.