I have been running a WiSM successfully for months now, but our wireless network has grown quite a bit so we have purchased a second WiSM.
Are there any best practises for implementing another one, or are there any design guides?
Your setup is basically going to be the same. However, you do want to make sure the virtual IP address on both WLCs on the 2nd WISM you purchased are configured the same as the other WISM if you intend to allow mobility between APs and clients. Additionally, you will also have to add your two mobility members to your existing controllers and configure them correctly on your new ones.
If you intended to configure manual load balancing you can use the primary/secondary/tert configuration on the access points.
All of the above is mentioned in the 4.0 configuration guide and it does cover multiple WLC implementation as well:
I hope this helps!
Here is a link to that good doc;
Cisco Wireless LAN Controller Configuration Guide, Release 4.0
Hope this helps!