We all want original melodies and ideas to trundle out of our heads fully formed when we write songs, but a genuinely good technique to use when this doesn't happen is to take whatever does come out -- be it cliched or sounding like someone else's song -- and then tinker with it, until it sounds different. That technique is even more effective with piano players, because coming up with new chords can be as easy as sticking your pinky out a half-step. Thnk of it as a carving-away & molding process, instead of an additive one. Most pop tunes are pretty similar in basic ways, so it shouldn't take too much to give a cliched tune a more original sound. Changing majors to minors here and there is a good start. Try to find the one chord that really stands out as the truely cliched moment, and then change it drasticly (to something that sounds good, of course), then go to the next particularly cliched moment, and alter that. Soon, you'll have an original tune. Use the same concept for words. Find the biggest cliche first, twist it to something unexpected, then find the next one, etc., etc...