Different polls use different methods, but most are via random telephone calls.

Here's how Gallup does it:

Gallup interviews 1, 000 adults nationally by telephone for the Gallup Poll Daily tracking program every day. The Gallup Poll Daily tracking updates track the well-being of U.S. residents 350 days of the year.

The Gallup Poll Daily tracking program is part of a larger project which interviews Americans on a continuous basis about their health and well-being across a range of income and conditions. The results are reported in continuous daily, weekly, and monthly averages.

The survey methods for the Gallup Poll Daily tracking program rely on live (not automated) interviewers, dual-frame random-digit (RDD) sampling (which includes landlines as well as wireless phone sampling to reach those in wireless-only households), and a random selection method for choosing respondents within a household. Additionally, Gallup Poll Daily tracking includes Spanish-language interviews for respondents who speak only Spanish, interviews in Alaska and Hawaii, and relies on a multi-call design to reach respondents not contacted on the initial attempt. The data are weighted daily to compensate for any disproportion in selection probabilities and non-response. The data are weighted to match targets from the U.S. Census Bureau by age, sex, region, gender, education, ethnicity, and race.

With inclusion of the cell phone-only households and the Spanish language interviews, 98%of the adult population is represented in the sample. By comparison, typical landline-only methodologies represent approximately 85% of the adult population.

FWIW, Obama is back up by +2 in today's Gallup Daily Poll.