It is the easiest thing in the world to forget. St. Francis de Sales uses the analogy that a blind man in the presence of a prince will show the lord great reverence when he is told that the prince is in the room, but will easily forget the great one since he can't see.
We are like that with God. He is always present and he always loves us. And we forget.
So, as we begin to pray, the first necessary thing is to "put ourselves in the presence of God."
And that means...?
It is not supposed to be anything long or elaborate. It isn't even prayer proper: it is the preparation we make before addressing God.
The best means I have found is first, simply, to stop. To stop physically from what I am doing and, mentally, from whatever thoughts occupy my mind. Draw a deep breath, naturally, and, if it is possible, close my eyes. Then simply think: "God is here," or "I am in the presence of God," or something similar.
Tomorrow I will talk about the four means St. Francis de Sales suggests.
Whatever means you use, however, it should be short, natural and (externally) not very noticeable. It should always precede your prayer time, be it long or short. The idea is that, if we want to speak to God, we need to realize that he is here, present, ready to listen and to respond.
Like any practice of prayer, doing it once is not the point. It must become a habit that we use all the time; something so automatic that we do it without really thinking about it. At first, it is conscious and "clunky," but over time it becomes natural and habitual. Sometimes you won't notice that you do it, but other people will, even though you made no show. A student once said to me, after a "routine" Hail Mary at the beginning of class, "Sister, you always pray like you mean it."
Well, at least I know that when I pray, I am talking to somebody. That is the reason for the practice of the presence of God.