something like this ("in cisco parlance" 
should do the trick:
router bgp 65534
bgp dampening route-map damp
!
access-list 101 permit ip any 255.255.240.0 0.0.15.255
access-list 102 permit ip any any
!
route-map damp permit 10
match ip address 101
set dampening 30 750 2000 90
!
route-map damp permit 20
match ip address 102
set dampening 15 750 2000 60
you can see the effects from the output below. in short, i've tightened the
thresholds on prefixes longer than /19.
lab2-7507#sh ip bgp fla
BGP table version is 91081, local router ID is 207.240.2.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
   Network From Flaps Duration Reuse Path
*d 215.1.0.0 207.240.2.2 4 00:02:35 00:58:30 65533
*d 215.1.0.0/20 207.240.2.2 4 00:02:35 00:58:30 65533
*d 215.1.0.0/16 207.240.2.2 4 00:02:35 00:28:40 65533
lab2-7507#
we're currently not dampening based on prefix length yet, although we are
experimenting with it.
-danny