ULPT: If you're a cashier/server/etc, you can increase the amount of tips you get by including more loose coins with the customer's change.

For example, give them four quarters instead of one dollar.

As many people prefer not to carry around lots of loose change, they're more likely to just drop it in your tip jar.

As an added bonus, there will be less coins in the register to count at the end of the day, if that's part of your job.