I played Tier II Junior A hockey for 3 and a half years... For two of those years my team was very strong and there were games when I'd have maybe 17 or 18 shots on me... It's a tough challenge to stay sharp in those games... The keys that I'd focus on are:
1) Keep yourself moving... Between whistles skate to the corners and back... stuff like that... Do some extra stretching in between whistles too so you will stay loose...
2) As CloudNine said, play a mental game during breaks... I had a goaltender coach once that helped me dramatically with the mental aspect of the game... One of the things that he taught me was playing the "mental games" during breaks... If there is a whistle (and you aren't skating into the corners
) put your head down, close your eyes and envision yourself making every possible save you can... envision a guy coming down on a breakaway against you and shooting high glove... envision the save... envision a shot from the point on the ice to the stick side and envision you making the save and controlling the rebound... it helps a great deal in keeping your mind in the game...
3) Play mini games! Another tip another goalie coach gave me was to break the game down into mini games... Instead of trying to escape from a game without allowing a goal or escape from a period without allowing a goal (which are both daunting tasks), try to break the game down into mini 3 minute games... Try to shutout the other team for 3 minutes... set small goals like that so you are achieving success throughout the game... it helps with your mental focus... sometimes you may not even get a shot during that 3 minutes... great... shutout!!! next game... 3 minutes goes by... 2 saves, no goals... excellent...
Unfortunately, the team I play for now (senior league) doesn't require me to use mental games to stay in the game... We are a fairly high scoring team but we tend to neglect our own end of the ice and I face a lot of shots, a lot of two on ones and a lot of breakaways... the other team keeps me in the game without my help