read the script, and mark it up for production sound.
Look for things like:
sonically challenging locations: water, heavy traffic, wind, etc. These things are often written into the script.
more than two people speaking in a scene: 3 or more people requires a certain amount of extra gear and/or coverage to get all of the dialogue right.
vehicle dialogue: different miking setups in these spots
running or other heavy movement dialogue: prep for wild takes here
talk to wardrobe
are actors providing their own clothes, or are they being produced? If there are costumes being created talk to wardrobe and try to minimize noisy clothing materials where others can be substituted. Wardrobe can also be of great help in creating mount points for lavs.
Plan out your post workflow.
How are you handling sync? If its timecode be sure that everyone is on the same page regarding frame rate/sample rate. If its plural eyes or manual with slate make sure your editor knows that workflow and won't hose you by cutting the picture before syncing it with the location sound. If you're running more than 2 channels make sure your editor handles all of the channels in post appropriately all the way through the time he hands you an AAF and a quicktime.
If you have the luxury of doing camera tests before primary photography, use that as an opportunity to shoot sound and run through your post workflow with the editor.
build your gear checklist
given all of that research, build your gear checklist. include all mics, cables, wind protection, recorders, mounts, and power. Then include non-audio things like carpet, blankets, gaff tape, etc.
talk to the director and AD
set up a meeting with the director and AD. Bring your marked up script and gear checklist, but don't throw those in his or her face and say "make this happen!" Instead, talk art for a while. Discuss the tone and vision of the piece. Talk about the general emotions the director is hoping to capture. Speak in broad terms and listen closely.
Then open the marked up script and go over any specific thoughts and concerns you may have given the context of the artistic conversation you just had. If you have a mumbly lead actor giving a dramatic performance planned in front of a waterfall or out in traffic, make the director aware of exactly how challenging that will be to catch audio for. Then work mutually on a solution or agreement on how to approach that moment.
Don't just discuss potential problems. Get the director on your side for potential high points as well. "This is cool, now it would be great in post if we were able to get the sound of xxx while were are there and set up. Can we build that into the shot sheet?"
acquire your gear
own what you have. Borrow what you can. Rent what you can. Build what you can. Have no excuses and have a gear plan for every shot you see coming.