These two matters are well worth attention:
First, whoever engages in prayer should apply to it his faculties and efforts, and not, as commonly happens, be distracted by wandering thoughts. For nothing is more contrary to reverence for God than the levity that mars and excess of frivolity utterly devoid of awe. In this matter, the harder we find concentration to be , the more strenuously we ought to labor after it.
Calvin, Institutes, III.XX.5