@KathyJudd You can set this at the web.config level so that you don't have to add ClientIdMode="Static" for every control you want to reference. See here. You can also set it at the page level.
3
After Rendering controls id will chaged something like cph_master_labelid
using jquery
var id=$(#'<%labelid.ClientID %>')
or You can Set its server side Property [Control.ClientIDMode][1]" Used only in 4.0 or greater