자주 쓰이는 방법이지만 외울 수도 없고 매번 검색하기도 귀찮아 포스팅합니다.

/* CSS Document */
/* here follows the brillant "no-extra-markup" clearing method devised by Tony Aslett - www.csscreator.com */
/* simply add the clearfix class to any containter that must enclose floated elements */
/* read the details of how and why this works at http://www.positioniseverything.net/easyclearing.html */
.clearfix:after {
    content: ".";              /* the period is placed on the page as the last thing before the div closes */
    display: block;          /* inline elements don't respond to the clear property */ 
    height: 0;                  /* ensure the period is not visible */
    clear: both;               /* make the container clear the period */
    visibility: hidden;      /* further ensures the period is not visible */
}

.clearfix {display: inline-block;}   /* a fix for IE Mac */

/* next a fix for the dreaded Guillotine bug in IE6 */
/* Hides from IE-mac \*/
* html .clearfix {height: 1%;}
.clearfix {display: block;}
/* End hide from IE-mac */
/* end of "no-extra-markup" clearing method */