Expires HTTP Header

.htaccess
ExpiresActive On

ExpiresByType text/html   A3600
# HTML expires in 1 hour

ExpiresByType image/gif   A2592000
# GIF  expires in 30 days

ExpiresByType image/jpeg  A2592000
# JPEG expires in 30 days

ExpiresByType image/png   A2592000
# PNG  expires in 30 days

# types not specified
ExpiresDefault "now plus 1 day"
#  expires in 1 day
Or, expire based upon modification time of document: 
ExpiresActive On
ExpiresByType text/html   M86400
# HTML expires 1 day after it was last modified
ExpiresDefault M86400