How can I improve on my sight reading? (piano)?

I have been playing the piano for almost 8 years now, and I can practically play any piece fairly well on my first try. The only problem is...I suck at sight reading. I have had a terrible habit of writing the letter (c, d, f, etc.) under each note or chord for as long as I can remember. If I don't write the letter under, I can't play very well at all. I was just wondering if there was any way to teach myself how to sight read quickly and without all the basics. (I have already tried asking my teacher for help, but she just tells me to practice chords on my own time). Thanks!