The title says it all. Instead of giving you the theory, here are two good examples. Instead of, “While he/she was walking, he/she heard a voice that said…”, try “While he/she was walking, a voice said…”. Instead of “There was always something that needed to be done”, try “There was always something to be done.” This is not something to obsess over, just think of it as a game. Start by writing freely, then revise and polish your text. You can also make a rule; for example, do not use “that” more than five times per page. This might be difficult but it will help you to write better sentences with better style.
Claudio Visentin and Guido Bosticco