Stretch your vocal range. One of the first things you will notice about every great rock singer is that he has a phenomenal ability to reach for the high notes. Perform regular vocal exercises that start at a lower octave and gradually work their way up until you're reaching for notes you can't quite get -- yet.
Imitate the greats. Practice learning to sing rock by copying those who are already famous. The trick is to not focus on one single artist, but on a group of artists. This will allow you to concentrate on different techniques that singers use, and won't train you to sound exactly like one person.
Put some "gravel" in your voice. Rock singers are often defined by their raspy vocal quality. It is almost essential to have that bluesy or raspy quality in your voice to be considered a rock singer. Play around with your voice quality when singing, and perform exercises like singing in a scratchier tone (without being annoying) or a smoky/breathy one to accentuate the songs you practice.
Play the memory game. When you are practicing a song to a recorded backing, stop the music every now and then and sing a cappella (without background music) to gauge your voice. You will then be able to better judge if you are on key. If you do not sound like the original to some extent, you are probably not on key.
Master your vocal abilities. Rock incorporates many different styles of music, requiring versatility on the part of the singer. This means being able to control your "instrument" as much as possible. Practicing your vocal licks and runs is a must; emulate soulful rhythm-and blues-singers when singing some rock songs. As you become more adept, flexing your vocal prowess and adding some flair to your style will help you to further push the envelope.