Start by getting a list of all of your neighbors name, address, and phone number. You can use mapquest or similar program to get started today.
Tonight, compose a form letter that you will mail out that outlines your position. Spell check it, grammer check it, let someone else read it and edit until it sounds direct and to the point. BE VERY PROFESSIONAL in the writing, or have someone else familiar with business writing help you draft the letter.
Tomorrow morning talk to the developer in person, get your facts in a row. Don't be hostile or rude, just ask what his plans are for the property. Document this, if you have a digital camera bring it along and take some pictures. Make sure to ask about issues like noise, traffic, crime, utilities, sewer, garbage pickup, and existing land use. Be honest and indicate your uncertain about changing the zoning for the property.
Once you have intel, use it to finish writing your letter. Then tomorrow night send each one of your neighbors a letter (well written professional letter printed on a laser printer) explaining the zoning change the developer wants, and why you think it's a bad idea.
Ask that they attend the next County planning meeting .
A day or two after sending the letters, either call them or better yet show up in person to talk to them about the rezoning. Make sure your clean cut, well dressed, shaved and washed. Good idea if you have a wife to bring her along as well.
Script out what your going to say ahead of time, and have a sheet with a summary of your position printed out that you can leave it with them. Advance planning in this case is going to be your best tactic.
Consider starting a petition by registered voters opposing the rezoning. Ask that they sign, but don't get upset or be pushy if they don't want to yet.
Ask that they attend the next zoning meeting so they will have a voice in what happens.
Make sure to call the local newspaper and have them send down a reporter to cover the meeting.
Nothing cools the jets of politicians faster than having two dozen pissed off home owners show up at their monthly meeting asking questions. Especially if they know their comments will be published in the newspaper the following day.