Court cases like Citizens United v. FEC and McCutcheon v. FEC, and the billions of dollars flowing into both state and national politics from unknown sources, have brought the issue of money in politics to the forefront. Congress can regulate donations to federal campaigns (see the Federal Election Commission for those), but each state has the freedom to set its own rule for state offices, within the confines of the Constitution as interpreted by courts, which prohibit limits on purely independent expenditures and on total spending by a candidate.
Within those constraints, the most frequently used tool to restrain the influence of money on elections are limits on contributions to political campaigns. Most states set limits on the amounts that individuals, political parties, PACs, corporations, and unions can donate to political campaigns, and in some cases, contributions from a particular source are prohibited.
Most states require contributions to be publicly disclosed, but disclosure requirements also vary by state, as does the form of disclosure.