i suggest not acing like you've got an unshakeable alliance and never getting into the position of being stalemated.
diplome, talk about balance of power. Tell one player you're willing to attack another to keep them from getting too powerful, and then do. Pull off if they attack you and threaten that you will let the other guy win. (your threats are more useful when you follow through.)
diplome the other guy aswell.
don't get into alliances with people who are so close to you that the very thought of attacking them means so many troops (because of your rather impressively long but unmanned border) you can't possible attack with 2 years movement and them noticing...
i'd refer you to : http://webdiplomacy.net/board.php?
gameID=14470but only becuase i'm proud of how the axis powers got to their agreed draw.
lots of troops along those borders almost all the time. (i'd say it would have gone to a stalemate eventually if we had stabbed someone, but as it's a league game we all wanted 4

and not being in front of all the other league players... we can risk the solos in the last game of our league)