From what I've learned there are some factors:
1) enough space to run
2) quiet, no sudden noises
3) patience
It's the #3 that gets me all the time. Our SLW was 24 weeks old when she laid her first on 9/3 and then the 2nd one today (9/5). Second egg was fatter than the first one. Just...
LOL. Actually, the first four were Catherine of Aragon, Lady Jane Grey, Anne Bolyn and Isabelle de Castille. Well, Lady Jane turned out to be Richard the Lionhearted and Isabelle was a Henry VIII. We rehomed them and they were renamed to Pepper (the SLW Roo) and Marshmallow (the EE Roo)...
Catherine of Aragon, my SLW (21 wks old) squatted in front of me today and i'm excited that i should be getting an egg within the next two weeks (hopefully sooner).