A tried and true approach to accurate sight singing is to become familiar with the first two notes of well known pieces of music and commit them to your accoustic memory. When you need to, recall the tune you learnt and hear the interval in your mind the next time you have to sing it.
Compiling your list of tunes could be a lot of fun, but could also be time consuming. This website –
– contains a list of well known pieces of music that start with the major and minor intervals of the diatonic scale. It also has a graphic that will show you the notes on a keyboard and on the stave. If you need to hear the music, there are even links to you tube!