I always try to find a bench or edge, upon which I could support the phone. If I really have to keep it in my hand, and I couldn't even support it on my knees (which is also works kinda well sometimes) I put a 2 seconds delay timer (I found that I put the most wobble in the phone when pressing the shutter button) and try to hold my hands as firm as possible. Even though I only use my phone when my camera isn't with me (and I have a tripod for that :D )