The phrase "et al." is an abbreviation and should always be followed by a period. It should be in the normal font like the rest of your document—do not italicize or underline it.
Example: Smith et al. (2010) found that drinking beer doesn't improve reaction time.
To type the example line in LaTeX (without using BibTeX) do
Example: Smith et al.~(2010) found that drinking beer doesn't improve reaction time.
The ~ will insure that there is a single space after "et al." and that "et al." and "(2010)" will be on the same line of the output. (I think having the period at the end of a line and the date starting on the next line is awkward to read.)
See The Chicago Manual of Style for more information.