On foot
On foot

There are many border crossings in urban areas which can be crossed by pedestrians. Crossings such as those in or near Niagara Falls, Detroit, Tijuana, Nogales, and El Paso are popular for persons wishing to spend a day on the other side of the border. In some cases, this may be ideal for day-trippers, as crossing by car can be a much longer wait.

By plane
By plane

Most visitors from outside Canada and Mexico arrive in the United States by plane. While many medium sized inland cities have an international airport, there are limited flights to most of these and most travelers find themselves entering the U.S. at one of the major entry points along the coasts:

From the east
New York City, Chicago, Philadelphia, Atlanta, Charlotte, Boston, Washington, D.C., Orlando, and Miami are the primary entry points from Europe and other transatlantic points of departure. All the major east coast airports have service from a few key European cities. Los Angeles and San Francisco, while not on the east also have a good number of flights from major European cities.
From the west
Los Angeles, San Francisco, and Honolulu are the primary points of entry from Asia and other transpacific points of departure. Seattle and Portland Oregon have a few international flight options. Of course, if you arrive in Honolulu, you must take another flight to get to the mainland. Foreign airlines are not allowed to transport passengers to/from Hawaii or Alaska and the other 48 states except for refueling and in-transit. If you are flying into the West Coast to transit to another destination, San Franscisco airport has a free frequent skytrain linking terminals and short security queues, compared to Los Angeles which will see you exposed to the elements catching a shuttle bus or walking between terminals. Chicago, while not on the west coast, is still a major point of entry from Asia, offering non-stop flights from Tokyo, Hong Kong, Shanghai, Beijing, and Seoul, with direct service from Bangkok and Singapore. Qantas serves Dallas/Fort Worth non stop from Sydney, in addition to their daily service to Los Angeles and San Francisco from Sydney and Melbourne.
From the north
Detroit and Minneapolis have a good number of flights from major Asian and Canadian cities.
From the south
Miami and Houston are the primary entry points from Latin America, primarily South America. Also, Dallas, Atlanta, and Charlotte are major international waypoints. From Mexico, most major U.S. airports have non-stop service.
From the other side of the world
New Delhi, India has non-stop service to New York via JFK and Newark airports and to Chicago. Mumbai has non-stop flights to New York JFK and Newark. From Pakistan, Saudi Arabia, Uzbekistan and United Arab Emirates you can also fly to New York JFK. Qatar, and Saudi Arabian fly to Washington, DC, and South African Airways goes to New York JFK and Washington, DC Dulles. Los Angeles and Houston both offer non-stop service to Qatar and the United Arab Emirates.

Note that the U.S. requires entry formalities even for international transit, and the current state of international affairs means that this is not going to change anytime soon. You must have a valid visa to enter the U.S. if required by your citizenship, even if you are immediately continuing on a flight to a different country in the same terminal. If your citizenship requires a visa to enter the U.S., avoiding a transit of the United States is necessary unless you want to spend time and money to obtain a C-1 transit visa. Further, when booking flights to the U.S., note that you will be required to clear customs and immigration at your first U.S. stop, not at your final destination, even if you have an onward flight. Allow at least 3 hours at your first U.S. stop.

Luggage allowance for flights to or from the U.S. usually operates on a piecewise system in addition to the weight system even for foreign carriers. This means that you are allowed a limited number of bags to check-in where each bag should not exceed certain linear dimensions computed by adding the length, width and height of the bags. The exact allowances and restrictions on weight, linear dimension and number of baggage allowed are determined by the carrier you are flying with, your origin if coming to the US or destination if leaving the US and the class of service you are traveling in.