I use a small script to attempt adding a bookmark using the most popular window methods, until all have failed. Then it just prompts the user to manually add their bookmark...
Like others have said (above) some browsers prohibit script-activated bookmarking, and because of security they want only users to add bookmarks.
It is not perfect, but it is simple and works well.
function addBookmark()
{
var success=false;
// try each until all fail...
try {
window.external.AddFavorite(window.location, document.title);
success=true;
} catch(e) {}
try {
window.sidebar.addPanel(document.title,location.href,'');
success=true;
} catch(e) {}
if(!success)
{
alert("AUTO BOOKMKARING not supported\r\nIn your current browser.\r\n\r\nPress CTRL+D, or CMD+D\r\nto manually bookmark this page.");
}
}