My thoughts on this (and please excuse any repition from others). I know the film has now folded but I think this is an important thread....
We shot Left for Dead over 18 months and our lead actors never once lost motivation, even though we never paid them or their expenses at the time. This is the way it was because we couldnt do anything else with it. But when they were at cannes a year later, watching their film in a room with sales people and seeing posters plastered everywhere they thanked us for the oprotunity we had given them.
1. Make them believe in the film. without that you might as well not bother. Make sure they know your aspirations, desires and wan't for the film. And make them /show them what you intend to do with it.
2. Be fair. Don't expect 16 hour days, no money and crap food. Treat people well and be nice with them on set. Shouting at a badly paid, badly fed actor in ac old room will prob get you punched or at least your actor walking out.
3. Get them listed on the IMDB with your film. That can help alot...
4. Non-Union actors who want to becomes union affiliated will often appricate letters detailing what they did on the film to help them get the accredition.
5. Press. Get there images & names in the papers (lcoal), film mags and across the web. All these thinsg will help with moral.
6. Give them good credits and make sure you ahev a contract detailing what they will get if the film amkes money. Again this helps no end.
7. Make it fun. have nights out, make the team bond and become friends because that will help no end.
Hope this helps...