First, the President is a citizen of the United States, and just like any other citizen is bound by its laws. As such, the commission of a crime is punishable under the appropriate judicial jurisdiction, as would be the case for any other citizen.
With respect to extra-constitutional uses of powers of the office, our balance of powers gives recourse to the Legislative branch through the impeachment process. As defined in the Constitution:
The President, Vice President and all civil Officers of the United
States, shall be removed from Office on Impeachment for, and
Conviction of, Treason, Bribery, or other high Crimes and
What constitutes "high Crimes and Misdemeanors" has some room for debate, but as Gerald Ford once said, it can be defined as:
whatever a majority of the House of Representatives considers it to be
at a given moment in history.
Articles of Impeachment are drawn up by the House of Representatives and require just a simple majority to pass. At this point, the Senate must vote to convict the Impeached individual and remove them from office. This requires a two-thirds majority of the Senate.
Examples of activity leading to Articles of Impeachment have included illegal activity, abuses of power, or simply activities unbecoming of the office.
Effectiveness depends a great deal on the observer and the degree to which they feel extra-constitutional activity continues to occur. In the history of the Union only 19 federal officers have been Impeached (not necessarily convicted or removed from office), however, whether that is an indication that the vast majority of federal office holders abide by the Constitution or that Congress tends to overlook such cases is up for debate.