Anyone who boosts is likely just wasting their time and the potential fun of the game. I don't give a damn about achievements so I don't boost. Where is the fun in that? You get a shiny mark on your gamertag. Woohoo.
I have only ever done one thing that could be considered boosting, and that was in LOTR Conquest for the 360. It had a horrible online fanbase and almost all the servers were constantly empty. So the first time a friend and I got online for a game, we noticed this and started off my screwing around. And we inadvertently 'boosted' a few smaller online achievements by pure coincidence. Seeing as no one was yet joining the server, after at least an hour of this screwing around, we decided to give ourselves a goal and just get the achievements. We didn't make it easy on ourselves really, we actually fought each other and didn't let one person kill the other on purpose or anything, but with two friends doing this alone it was pretty easy to get everything done in due time.
Other than that, I just fuck with anyone I find trying to boost. Like in Halo 3, I would enter those lone wolf games and whenever everyone else would agree to help get some of the harder achievements, I would pretend to comply for a bit before screwing with them all and making it into a real game. And never once did I get an achievement from it myself, hehe.
Ace Combat 6 was another example. People wanted to boost achievements for battles won all the time. This was easy to combat though, all I had to do was play. And I wiped the skies clean easily on my own or with my team. Other people would try to screw with stats in ranked matches to get in the top 100 for each map so they could have an "Ace" designation next to their name ingame. I would go in and make sure they couldn't boost that top score by either destroying them in combat or just flying away and keep them from getting any points at all.