One simple way to keep people from voting is to require them to pay a tax for that right, and to make it just high enough that much of the population can't afford it. In the early 20th century, most of the former states of the Confederacy imposed such poll taxes. The amounts weren't that high by contemporary standards — Virginia charged $1.50 per year, about $11 in today's dollars, while Mississippi charged $2 (about $15 today). Even so, the taxes had to be paid in cash, which amounted to a prohibitive hardship for sharecroppers, miners and small farmers, who generally bought food, clothing and other necessities with credit, and never had more than a few spare dollars in their possession.
Additionally, in Virginia and other states, the taxes were cumulative, which meant that a prospective voter had to fork over the cash for several years in a row before being eligible to register. Other places cut down on participation by only sending out notices about the tax to property owners, or requiring voters to show up at the local sheriff's office, which served to intimidate poor people and minorities. In 1964, the ratification of the 24th Amendment to the U.S. Constitution prohibited poll taxes, but it wasn't until two years later that the last four remaining state laws were struck down in federal court [source: DeSilver].