Anyone know how to do this? I want to sing and play at the same time, I find that some songs I can do it (the simpler the rhythm the easier), others not so much. Any tips on how to practice this or get better at this? 

Nothing really particuliar to do. It's way harder than simply playing or singing and practice is the only solution here. Also the more you know by heart the guitar or lyrics the easier it can be since you need less concentration on the part you know so working one part alone can help too.

 

On some songs where rhythm and lyrics doesn't match well together it can be really hard to do it correctly.

Thanks so much firekorn  :D . There are a few I can do with no problem. The really well done 21 Guns - Green Day CDLC is an easy one (rhythm or lead guitar), Black - Pearl Jam (rhythm). I'd like to play more complex stuff and sing, but I think it's like you said, practice...there are no shortcuts I guess huh? :(

All I could tell you is that at some time I started to be able to do so, first with easy songs and with practice (Doing the same with hundreds of tracks) I became good enough to do the same with the more complex stuff. Just keep going at your own pace and you can get to it.