There are always a few people who lie on surveys but the anonymity prevents a lot of that. If the survey base is big enough, it's insignificant.
Pew Research does really good work. They're independent, no one hired them to do their studies so they don't have an axe to grind. In 2007 they did a study on religion in America. 35,000 respondents.
71% of Americans are absolutely certain that there is a God.
58% pray at least once a day and here's
daily prayer by religion, age, gender and income.
ETA: I just clicked on the links. For the first two, they lead you to the main menu for the study. Use the "select a topic" drop-down under the map on the right.