Besides [the great post](http://rpg.stackexchange.com/questions/31741/how-to-voice-an-npc?rq=1) that [@kviiri already mentioned][1], I would like to add that you could use *well-known stereotypes*. Stereotypes are good because they help to remember and to roleplay the behavioural characteristics of an NPC, but also because they make it easier for the players to relate to the different diosyncrasies.

For instance, if your NPC is a sailor, it might help you to think of him/her as [Popeye][2] (even if the NPC is not that character at all!) while roleplaying, by trying to include some of the tics you have seen on that famous character before.


  [1]: http://rpg.stackexchange.com/questions/81011/how-do-i-give-my-npcs-personality?noredirect=1#comment183220_81011
  [2]: https://en.wikipedia.org/wiki/Popeye