I use CS3.. but their pretty similar .. the way i do it.. go to select.. and choose color range "its near the middle of the drop down menu".. then i select the background colors and you can adjust this by changing the fuzziness.. less means the color range will be less .. and more means that the range will be greater.. also if you hold down shift you can select multiple colors at once.. Its easiest if you use a picture where the background colors are different then the image you want to keep.. but if you thats not the case this will still work but you will have to use the lasso tool to deselect parts of the image that were the same as the background..
To help smooth the picture out.. i do this last.. get the background you want in and then use the blend and smudge tool to.. make it look more natural... Also using layers and layer masks properly can help too..
