Can I increase caching?

[Steve Snyder]

I'm running a nameserver (BIND 9.2.4) on a CentOS 4.1 system.  My nameserver 
is authoritative for my own small zone and caches general internet name 
resolutions.

My configuration is generally working well, but it seems to have a high 
percentage of recursions relative to successes.  For example:

success 13037
referral 0
nxrrset 2972
nxdomain 3420
recursion 11334
failure 2658

(I believe the high number of failures is due to attempts to crack my 
firewall.  Secure SHell attempts to resolve every connection that rattles 
my door knob.)

Beyond what seems to be a low number of cache hits, I have unused memory on 
my nameserver machine.  That is, system RAM that is not being used for 
anything, even disk caching.  I'd like to spend some of it, if possible, on 
greater caching of resolved domains.

Any thoughts on what I could do to increase caching of data by BIND?

Thanks.