You just can't do it: you save a kid's life, because you think that will increase overall utility, and the kid turns out to be Hitler. You shoot someone beating an innocent victim, and it turns out the person was going to reform, and wind up curing cancer. No one can possibly calculate what actions will "increase overall utility," and what ones won't.
So what utilitarianism as a matter of fact is either:
1) A way to justify doing whatever it was you wanted to do anyway: just put in the right consequences, the right imaginary futures, etc., and you can make any action look good!
2) Or you opt for rule utilitarianism, which just winds up meaning that you should follow the rules of morality that you knew you should follow all along, before you ever started mucking about with utilitarianism.