Don't make assumptions. Remember, not everyone buys SUVs for the same reasons. Sure, there are clueless bigwigs who like to rub their affluence in other drivers' faces, but some families buy SUVs because they have lots of kids (it's hard to fit seven or eight people in a Corolla). Believe it or not, there are even folks (though not as many as carmakers would like you to think) who buy SUVs because they engage in all those fun off-road activities portrayed in TV commercials.
Express your opinion. Not everyone has the time (or the inclination) to ponder the environmental impact of his lifestyle. If your neighbor is considering upgrading his mode of transportation, chat him up about global warming, depleted oil reserves, and the fact that America's addiction to gasoline helps to finance repressive regimes in the Middle East. (But be casual about it-presenting a printed manifesto may result in a fistfight.)
Set an example. Talk is one thing; action is another. If you're in the market for a new car yourself, put your money where your mouth is by purchasing a hybrid vehicle (which is partly powered by electricity) or a small, but safe, car that gets good gas mileage. Whatever you do, don't feel you have to buy a big car to compete with other SUVs on the highway (though, sadly, many people do trade up to SUVs simply as a matter of safety).
Get political. There are numerous environmental organizations that include some kind of "anti-SUV" plank in their basic platforms. Lending your support is a way to influence policy makers directly, so that Congress, say, can be persuaded to pass more stringent gas-mileage guidelines, or to provide financial incentives for consumers to "trade down" to hybrid cars.
Don't break the law. Nobody has the right to deface another person's property: the only message you send when you slash an SUV's tires is that you're a sociopath who deserves to spend time in jail. It's also not okay to slap one of those extra-sticky anti-SUV creeds on a vehicle's front or side windows, which can render the SUV impossible to drive (look at it this way: if the owner has to rush her child to the emergency room, do you want her to waste precious time dealing with your vandalism first?)