Men should wear undershirts to work -- generally crew neck t-shirts. If you button the top button of your polo shirts, you should wear a v-neck t-shirt in this instance. With respect to tucking, button your pants and then pull your undershirt and shirt down straight and tight by accessing them through your fly. If your t-shirt and shirt don't at least cover your privates, you need to try some different sizes and/or brands than what you're currently using.
Good advice. After reading this thread, I also recommend the Ask Andy forums as a decent place for generic men's clothing questions.