I realize that while Bush is moderately popular here, he is wildly unpopular in other countries. While I want to encourage people from other countries to voice their opinion, they also have to be very respectful of people here if their letters are to do what they hope they will. This goes for US residents who are trying to convince US voters in battleground states also.
Some key points to consider in such letters are:
1. Be respectful to who you're writing to. Any show of disrespect may make a US voter more likely to vote for Bush out of spite.
2. Be respectful of the intelligence of any US voter. Even if you're convinced they're a brain dead hick, implying in any way that they are is not productive in the slightest and will only go against what you're trying to accomplish.
3. Don't start telling a US voter their own history as if they don't know it. They may not know it, but the odds are that they are more familiar with it than a foreigner is on average. You're just likely to insult them even if you do know US history better.
4. Don't tell them how badly Bush is ruining the US. People tend to believe that they don't need a foreigner to evaluate their own country. You'd just be insulting their intelligence. Rather tell them about your country and how Bush affects you and your country. If you're in India, you might praise Bush for making outsourcing jobs to them so much easier.
5. Don't threaten a US voter in any way. That will just turn them off and they'd be more likely to vote for Bush.
6. Realize that the US voter can vote anyway he or she chooses and you can't threaten or force them to vote in a different way. All you can do is be respectful and plead with them not to vote for Bush.
If you have any points to add, I'll add them to the list and give you the credit for it.
The most key battleground states are
Florida (27 Electoral Votes)
Ohio (20 Electoral Votes)
New Jersey (15 Electoral Votes)
Iowa (7 Electoral Votes)
Nevada (5 Electoral Votes)
These are states that are the most likely to be swayed either way. While Florida is critical, its citizens have more senior citizens in it than other states and you have to be especially respectful there.