proc incr_label {w} {
  $w getValues -labelString x
  incr x
  $w setValues -labelString $x
}

xmRowColumn .rc managed -orientation horizontal
xmPushButton .rc.btn managed \
    -labelString "increment label"
xmLabel .rc.label managed -labelString "  1"
.rc.btn activateCallback {incr_label .rc.label}
