Support is a big thing and one of the reasons I left my class as I got NONE from my dreadful leader. Texts, facebook pages, events, it all helps to feel the members feel part of something and motivated to continue.
Also, don't just make every meeting about going round the group and asking how people have done. Whilst thats fine every 2/3 weeks it gets so boring, you don't learn anything. Make the meetings fun, pick a theme to discuss and bring in props, get everyone involved. SW really falls down on the meetings, I was a WW leader for 2 years and every week we had to write a 30 min meeting that got the members involved and learning something - it was a pain for me(!) but my members loved it and it helped with their motivation no end. For example, one day we discussed the calories you burn doing housework which lead on to body magic discussions. We did recipe swaps but I had everyone on their feet going round to other members to show them what they had. Other weeks we discussed the best 'junk food' or chocolate to have but the emotional reasons why we craved it too. I've still got all the stuff saved on my computer and go back to it from time to time for motivation, even though it's a different diet the messages are the same!
Good luck!