I used to only contribute at conventions/assemblies (but I definitely contributed WAAAAY too much there) and maybe like once a year I'd drop a $20 in the box at the hall. The congregation I was in was always well off and constantly giving money to the branch so I didn't feel like they needed it. I'm ashamed to say I was sucked in by the perpetual deficit at the assemblies though.
About 4-5 years ago, though, they simultaneously did a huge (unnecessary) reno of the assembly hall and rolled out the credit card machines. Either one of those individually would've been enough to make me stop contributing. Never gave a dime since. I figured if they're just blowing the money on useless crap, I might as well blow it on useless crap for me!