Using LDAP over SSL is a good step towards security. Improve security just a little bit more by disabling SSLv2 and forcing your clients to use SSLv3

On each of your domain controllers create the following registry key:

\SCHANNEL\Protocols\SSL 2.0]

Then create the following DWORD

DWORD = "Enabled"
Value = 00000000

Finally, reboot the domain controller

To make this even easier you can deploy this registry key though a Group Policy Object linked to the Domain Controllers OU.

More information from Microsoft: How to Restrict the Use of Certain Cryptographic Algorithms and Protocols in Schannel.dll