One classical definition of prayer defines it this way: "Prayer is lifting mind and heart to God."
That's a wonderful and accurate description of prayer, the problem is that we rarely do that. It's rare that we actually open mind and heart to God in order to show God what's really there. Instead we treat God as a parental-figure or as a visiting dignitary and tell God what we think God wants to hear rather than what's really on our minds and hearts.
As a result we have a pretty narrow range of thoughts and feelings that we consider suitable for prayer. Most of what we actually think and feel is considered too base for prayer. We feel we are praying only when we have attentive thoughts and warm feelings, when we feel like praising God, when we feel altruistic, pure, centred, when we have good feelings towards God, others, and nature, when we feel the desire to pray more, or when we yearn for moral improvement.
Such thoughts and feelings do make for prayer, but we can't turn then on like a water tap. Many times, perhaps most times, we experience other thoughts and feelings: boredom, tiredness, dissipation, bitterness, sexual fantasy, and sometimes even a positive distaste for church, prayer, and moral improvement. We don't feel that it is valid to lift these bitter thoughts and impure feelings to God. Instead we try to crank up the thoughts and feelings that we think we should be having when we pray.
There is some legitimacy in this. Classically, spiritual masters have distinguished between prayer and distraction. Prayer, they point out, requires an effort of concentration, of attentiveness, an act of will. It isn't simply daydreaming or letting a stream of consciousness occur.
But prayer is "lifting mind and heart to God" and that means lifting up, at any given moment, exactly what's there and not what, ideally, might be there. It would be nice if we always felt warm, reverent, altruistic, full of faith, chaste, hopeful, connected with others and nature, happy about who we are and what life has dealt us. But that isn't the case. We all have moments and even seasons of doubt, anger, alienation, pettiness, boredom, obsession, and tiredness. Our thoughts are not always holy and our hearts are not always warm or pure. It's at times like this we need prayer and what we need to take to prayer is, precisely, those bitter thoughts and unholy feelings.
All thoughts and feelings are valid material for prayer. Simply put: When you go to pray, lift up what's inside of you at that moment. If you are bored, lift up that boredom; if you are angry, lift up your anger; if you are sexually obsessed, lift up your sexual fantasies; if you are tired, lift up that tiredness; if you feel selfish, don't be afraid to let God see that. Jesus said that we must become like little children to enter the kingdom of heaven. One of the qualities in children to which this refers is precisely their honesty in showing their feelings. Children don't hide their sulks, pouts, and tantrums. A good mother handles these rather easily, often with a smile. God is up to the task. In prayer, we can be transparent, no matter how murderous, adulterous, or irreverent our thoughts and feelings might seem.
If we do that, it makes it easier for us to "pray always", as scripture asks. What does this mean? Obviously it doesn't mean that we should always be at formal prayer, that we should strive to be full-time contemplatives, or even that we should seize every possible occasion we can to pray formally.
To "pray always" invites us rather to live our lives against a certain horizon. It doesn't necessarily mean to stop work and go to formal prayer, important though that is at times. The point is rather that we need to do everything within the context of a certain awareness, like a married man who goes on a business trip and who, in the midst of a demanding schedule of meetings and social engagements, is somehow always anchored in a certain consciousness that he has a spouse and children at home. Despite distance and various preoccupations, he knows that he is "married always". That awareness, more than the occasional explicit phone call home, is what keeps him anchored in his most important relationship.
Our relationship with God is the same. We need to "pray always" by doing everything out of that kind of awareness. Moreover, when we do spend time in formal prayer, we need, like children do, to tell God exactly how we feel and invite God to deal with that.
Rabbi Abraham Heschel points out how, in prayer, the great figures of scripture did not always easily acquiesce to God and say: "The will be done!" They sometimes fought bitterly and said: "Thy will be changed!" That can be good prayer. It lifts mind and heart to God.