A sight will not help you, if you have to aim high on uphill shots it is because your form is going to pot as you raise your bow. Uphill shots are the same as down hill shots, the arrow will arc less, and will normally hit high, meaning you have to aim low. the speed of most modern bows negates almost all of this, and for the most part you can aim dead on.

If you are having problems like you stated it is most likely form related.