One extremely important thing to remember is that you can never become someone else. An actor is always his/herself, and only when the actor realizes this can they live truthfully under the imaginary circumstances of the play. These circumstances might require a certain walk or a certain dialect, so it's a good idea to use these often, if not always while in the space. I find it useful to choose an independent activity that the character might do alone in their spare time, and do this activity before rehearsals and performances. Other than that, it's always a good idea to review objectives and as ifs for individual scenes.
When reviewing lines (either to yourself or to a partner) you should always try to change the way you say them, even if you think it sounds absurd. This is a way of freeing up the voice, keeping away from specific line readings, and getting the creative juices flowing.
I guess the short answer would be this: There is no way to really "get in character." Preparing to act is about loosening up, and making the necessary accommodations vocally, physically, and emotionally.