Too often, I have been told by people that they really do agree with most of what the average Libertarian candidate says, but they just can't bring themselves to vote Libertarian (or any other third party) because that candidate has no chance of winning, making it a wasted vote.
If every person who has ever told me that actually voted Libertarian, there would be some radically different outcomes, and you wouldn't see it as a wasted vote. You would wish you had done it sooner.
If you do largely agree with the LP, but are picking Bush or Kerry, or any Democrat or Republican instead of voting Libertarian because who you are picking is not his opponent, you will be sorely disappointed if your fall-back choice does win. Small-government voters who vote Bush instead of Badnarik will get bigger government anyway. Anti-war voters who vote Kerry will get war anyway. Etc.
Worse, you will have sent the message to the parties that they don't have to change a thing. They have your support, and the proof is your vote. In fact, the only way to tell them that they need to change is to vote Libertarian. In fact, the only vote that is truly wasted is one cast for a candidate you can only begrudgingly support.
Indiana's gubernatorial race is exciting for the object lesson that will come of it. Democratic incumbent Joe Kernan has stated his unwavering support for the construction of a new section of I-69, through new terrain, including wetlands. Republican challenger Mitch Daniels agrees with Kernan. There are many voters who hold the environment as their number one issue, and they normally vote Democrat. These voters are deeply disappointed by Kernan's position. Some will waste their vote, and support Kernan anyway. Those who are disgusted enough by Kernan will vote for Libertarian candidate Kenn Gividen, the only candidate to oppose the new highway. The message will be sent to Kernan and the Democrats that environmental voters must not be ignored. When Kernan loses by 1-2%, and he sees that he lost 3% to Gividen, he and his party will get it. No longer will Democratic candidates for Indiana governor ignore the environment.