The first issue is to understand how the game can end. There are three basic ways that game can end and each has an impact on your final score. The first is that you can be the first to play off the last of your tiles. This provides you a substantial bonus. Your final score is increased by the sum of the points of your opponent's remaining tiles. Furthermore, your opponent's score is then reduced by that amount as well. This gives you an effective addition of twice those points. The second way the game can end is the exact opposite. Your opponent plays off their last tiles and they net the advantage to your detriment. The final way occurs if neither player can empty their rack. In this case, each player deducts the some of the points of their remaining tiles from their current score. At this point, the higher score wins. So it is quite possible to be trailing during the game, but finish ahead if your opponent has more points on their rack than you on yours.
Knowing this, the goal remains the same - to outscore your opponent. However, understanding these nuances will be key. Throughout most of the game you are trying to maximize the points you score by placing words on the board. However, at the end game, this is not sufficient. At this point, you will want to maximize your final score after each turn, subject to some considerations. Say you have a two tiles left, a 1 point tile and a 4 point tile. Perhaps that 1 point tile is an S and the 4 point tile is an M (these are Words with Friends tile points). Quite possibly you could score quite a few points with the S, by perhaps pluralizing two nouns. I've done this several times. Say you can score 14 points by doing this. On the other hand, you might be able to only score 10 points playing the M. If this was your last turn, the play with the M is actually better. Since your effective net benefit from the S is the 14 points scores less the 4 from the M less the 4 again from the M as they are added to your opponents score. This then gives you a net gain of 6 points if your opponent clears their rack on their next turn. In contrast, if you play the M you get the 10 points there then lose 1 for the S and lose 1 again as the 1 is added to your opponent's score. This is a net gain of 8.
So why might a player not want to try to even maximize their final score in the end game? Quite simply, they might have been keeping track of tiles and know their opponent is stuck with some horrific tile that is not possible to be played. At this point, your plays begin to depend on the current relative score. If you are ahead, you simply need to close out in an efficient way and continue to trap your opponent's tile on their rack. If you are behind, you need to try to make up the score difference and again keep your opponent's tile trapped. This latter situation might call for making as few plays as possible. For example, in the end game in one of my recent games. My opponent played a K with three squares to the triple word score. I had an I, T and an E. I could play KITE and score 24 points. However, I could also try to milk my score, by first playing KI for 6, then playing KIT for 7 and then adding the E for 24 more points. Furthermore, at this point if I had some other letters, say an M, I could have played ME and KITE with both on the triple word score. The first level created an extra 13 points. One can quickly see how this is beneficial. Trapping a tile on your opponent's rack puts you in a powerful position to score extra points and possibly come from behind to win the game.
So there are some keys to mastering the end game. This applies to Scrabble, Lexulous, and Words with Friends.