Some gravity helps, as well as experimenting with and without a pick. I also have found practicing strumming in a subconscious way (watching TV or distracting yourself) can make it much easier to physically get more comfortable and a singular sound. I can do a PT 2 to this eventually to give more tips.
My strumming has been coming along pretty good so far (5 months). I'm using a classical guitar, no pic. But I've been working on the song 'The Highwayman'. I have the chords down okay and the tune. What I can't seem to find is a good strum for the song. I'll keep trying but it's frustrating.