Find a good script that is very well-written. Even if your financing is in place, the quality of your script will go a long way in encouraging the star to get involved. If you need the star on your picture to get the financing, a script that "lives" on the page is even more essential.It also helps to have crew talent (including a director, DP and producer) attached.
Get the script directly to the star under circumstances that will prod him to read it fairly soon. Who do you know who knows someone else who knows the star? You don't want to simply drop the script in the star's mailbox in the dead of night (although some have tried this). You want it to get into her hands with some provocation to read it. If you succeed at this, make sure to immediately follow up with a call to the agent to say, "I ran into Jodie at an AA meeting last night and gave her my script. I just wanted you to know that she has it...can I send you a copy?" Agents hate it when you go around them. It helps their egos to be able to say, "Oh yeah, I knew all about that."
Build a reputation as a film producer. Build relationships with agents and managers. The more well-known you are, the easier it is to get agents to send scripts to clients. In every step of the agent-to-manager-to-star chain, you want to have the most positive exchanges possible. Keep it personal if you can. There's nothing stronger than a credentialed person showing passion over a project. If you are not yet credentialed, see if you can get someone with clout to submit the script with excitement.
Get a well-connected casting director to submit the script on your behalf. This is a good approach to consider If you do not yet have the proper relationships and reputation. You might have to pay the casting director or offer an expanded credit (like associate producer), but if you get the talent, you may well be getting a chance to make your movie that you wouldn't otherwise have had.
Don't bluff. You want to keep the star on your side. If you are using "A" talent to get financing, you had better have done some homework in putting the financial pieces together. With the star(s) you have secured, can you feel reasonably sure that financing and distribution will follow? Hollywood becomes a very small town very quickly. Tell the truth.