Founder of the Mont Alto Film Project
Presumably we are talking about no/low budget projects. For well-funded projects you would want to work with a energetic, well-connected casting agent.
The system I use involves every resource at my disposal. I always hold auditions and will not cast an actor until I've seen him/her perform on camera and until I've had an opportunity to see if s/he can take direction. Because I am working with micro-budgets and am not in a heavily populated area, I insist on actors showing up for the audition. If they can't make the audition, then how can they make the shoot?
That said, I use multiple resources. I post the audition notices on every acting website I can find including mandy and backstage. I also post flyers at every college or university within a reasonable driving distance (1 1/2 hours is about my limit). In addition, I usually contact someone in the drama/acting department so I have a personal contact inside the schools. Because I am about an hour away from Baltimore and 1 1/2 hours from D.C., I also hit up acting schools/sites in those areas.
In addition, we post flyers everywhere we can: restaurants, local community bulletin boards, and so on. You never know where the best actors are.
To help matters, I try and personally respond to every actor that contacts me, if their headshot and resume look like they might be appropriate for some role. I specifically ask if they are planning on attending one of the auditions. I don't, as I've said, let them send in a video audition unless I already know them or they just seem perfect for the role and they live a zillion miles away. This to me is very important because if is not enough for them to look right and act well, they also need to be reliable and cooperative.
By the time all is said and done, I have posted audition notices everywhere and probably reached a few thousand actors and wanna-be actors. Then I hold professional auditions and we see who turns up. If all goes well, I can cast significant roles from those, but I seldom find all my actors from the open auditions. For the current film, we held three open auditions on three different days. Each ran 4 hours and we auditions perhaps 120 or so actors. We had about 18 roles to fill and from the auditions we cast 12 roles, including 3 of the 4 leads. For the remaining roles, I go to acting websites (usually in Baltimore or D.C.) and start inviting actors with the right look to come audition. These are individual auditions that I usually hold in my home, but I still put everyone on camera and run them through the same process as in the open auditions. If all goes well, I at least have my main roles cast. If I have featured parts (1 day of shooting), I start looking for people who I know or someone I know knows and start asking if they want to audition. That is where I find the stragglers.
That's my process for finding actors.