first of all get the guitar down so you know it without lookin at the fretboard (if you can, sometimes there are parts where u kinda need to see the fretboard to be accurate)

second of all put the song on and play along, now sing during the parts of the song that are easyiest to sing by (like the constant rythm parts) hum the rest

that seems to work for me.

somebody on another forum told me that it wasn't something easy to master and i'd have to make a big effort each song i play, however i can sing along to any song i know now adays. a Hint is turn the song your playing up way loud so you can't even here your voice, that way you're not distracted by your voice, and you can hear it really clearly, then later on as you get used to it, you can do anything.

these are just my suggestions, they've worked for me and i've been playing about 6 months (3 months intensively) and i can do basic punk songs which are powerchords or riff based songs, and songs like Sanitarium by Metallica whilst singing.

It's easier to learn with consistant, repetitive, riffs that don't move around the fretboard too much.