debouncing switches with a timer interrupt?

[DDDvvv]

hi, again, im trying this piece of code, from a previous post

RB_SUBMenuLoop:
' BTN_PLUS is the Next Choice button
   IF BTN_PLUS=1 THEN
      BTN_PLUS=0
      ...
   ENDIF
' BTN_PREV is the Exit button
   IF BTN_PREV=1 THEN
      BTN_PREV=0
      RETURN
   ENDIF
' BTN_NEXT is the Goto SubMenu Choice button
   IF BTN_NEXT=1 THEN
      BTN_NEXT=0
      ...
   ENDIF
GoTo RB_SUBMenuLoop

then i have this code scanning port b, through a 1ms interrupt:

Changed = PortB ^ OldPort
         IF Changed.4 = 1 THEN     ; Pick your pin
         OldPort = PortB
         BTN_PLUS = 1
         ENDIF

this code is incrementing one at a time, which is what i want, but the only problem is that it is incrementing on a rising and falling edge of a button push. so if i push the btn_plus, bmenupos increments by 1, and if i release it, bmenupos increments again.

i want it to increment only on the rising edge, not on the falling edge. any ideas?