css aktiv menu

Jeg er igang med at lave en menu til et site i .asp
Hvordan skriver jeg css så at menuknappen forbliver aktiv på den valgte side. Her er min css kode. 

/* menu-top */



#menu {
    margin-top: 0px;
    height: 46px;
    width: 955px;
    background-image: url(/Resources/Files/images/menu-bg.jpg);
    background-repeat: no-repeat;
    padding-left: 40px;
}
.menuseparator {
    width: 0px;
    height: 0px;
    float: left;
}
.menuitem {
    height: 46px;
    display: block;
    float: left;
}
.menuitem span {
    display: none;
}
#menu1 {
    width: 84px;
    background-image: url(/Resources/Files/images/link1.jpg);
}
#menu1:hover, #menu1-active {
    width: 84px;
    background-image: url(/Resources/Files/images/link1-active.jpg);
}
#menu2 {
    width: 127px;
    background-image: url(/Resources/Files/images/link2.jpg);
}
#menu2:hover, #menu2-active {
    width: 127px;
    background-image: url(/Resources/Files/images/link2-active.jpg);
}