Here is my list of how to get A-List actors onboard:
1. Become very good friends with them!
In all seriousness! You may not become great friends iwth them, but you can at least become aquaintances. Go to film festivals, go to parties, be at events, go out. Over time, you will develop a relationship with them. This becomes very important. But, a caveat: NEVER TALK BUSINESS! They get it all day every day. Find something else to talk to them about. Talk politics, ask them where they got that shirt, tell them about the best burger in town that's located in the most obscure hole-in-the-wall joint, ANYTHING, but business. Schmooze is the word!
2. Become very good friends with somebody who is very good friends with them.
This is probably the best way to do it. An agent will rarely take a look at indie films if it is a MAJOR star. Maybe look into A-Listers who haven't really broken out to be come huge stars, or haven't done a picture in a while. The first is like some Scarlett Johannson films over the past few years. The latter is what Tarantino did when he got Keitel onboard with Reservoir dogs.
3. Stalk them (Kidding! I'm kidding!)
4. Write the part to be a supporting role, but a major emotional role.
This is so that the actor really wants to do the part. Also, because it is a smaller part, their time is not constrained as much by schedules and they have a better chance of being available to do it.
5. Write a great script (obvious)
6. Talk to the people funding the project, or look for actors who identify with the theme.
Kal Modi did this recently. You may recognize him as Taj from Van Wilder and Kumar from Harold and Kumar Go To White Castle. He recently did a SHORT indie film called "American Made" (which I highly recommend). While this is only speculation, I can bet the fact that it dealt with being an Indian in the US today, as well as the Indian heritage of the filmmaker and financial backers played an important role.
7. Find a connection with somebody.
I know that many people like to come back to their roots. For instance, I live in Kansas. If I wanted to, I would go to people like Chris Cooper who grew up in Kansas City, Dennis Hopper who grew up in Dodge City, or even talk to Dan Glickman (now head of the MPAA) whose family still lives in Wichita.
8. Go to film school.
You might make connections here. But, I digress, I talk about this in another thread.
9. Go to film festivals regularly.
Become a staple. Be the guy that everybody expects (and, preferably, wants) to see at film festivals. THis goes back to number 1.
10. Become the next Steven Spielberg, Quentin Tarantino, or Kevin Smith.