There are several reasons that a government might declare martial law and each reason would probably require a different type of preparation.  Recently our govt has passed laws allowing them to take control in times of terrorist attack.  Martial Law can also be declared in times of civil unrest or times of war.  During times of martial law in the United States the basic right of Habeas Corpus is suspended and according to the Constitution

"Article 1, Section 9 of the U.S. Constitution states, "The privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion, the public Safety may require it." - Wikipedia

Martial Law has been declared in the U.S. once and that was by Abraham Lincoln during the Civil War which was later found to have been unconstitutional.  Martial law was also declared in Hawaii during World War II and in Boston during the Boston Tea Party by our good friends the British.  At the time martial law was declared neither was part of the United States.

FEMA camps or some like to call them prisons for Americans

Apparently there are camps being set up all over the U.S. and the only explanation for these large camps are to hold large amounts of people.  Here is a list of supposed FEMA potential camps and their locations.  My biggest questions is who are they being set up to hold?

American Troops doing stops and searches on American soil

Below is a video which shows American Troops on American Soil practicing doing traffic stops and taking over American Streets.  While the commentator is obviously slightly bias it is surprising to see American Soldier doing live searches on American Streets.  There are plenty of other places to train for that and the fact that they practice taking over the entire neighborhood is a bit frightening to us.

FEMA is training U.S. troops to repond to domestic issues - Operation Garden Plot

This is a pretty serious video with some pretty good documents and hard evidence to back up their claims.  It's a little dated but a good video.

Government to rely of clergy for help in case of martial law

Below is a news story documenting how the government with use American Clergy to control the American population if Martial Law were ever to be declared.

We have collected all the survival equipment you could need to survive any declaration of Martial Law.  We have collected all the survival equipment in one convenient location for your shopping needs.  People don’t have the time to do the research and find the best products so we have done that for you.  Below are our favorite items for when and if Martial law is ever declared. You can find a more complete list of our products in our survival supply store.