We do something like this.
Code:
<!-- yQuard obfuscation task definition -->
<taskdef name="obfuscate" classname="com.yworks.yguard.ObfuscatorTask" classpath="${yguard.home}/lib/yguard.jar"/>
<echo>Obfuscating the code</echo>
<obfuscate logfile="${build.dir}/obfuscation-log.xml" conservemanifest="true" replaceclassnamestrings="true">
<property name="language-conformity" value="compatible"/>
<expose>
<class classes="protected" methods="protected" fields="protected"/>
</expose>
<inoutpair in="${build.dist}/${ant.project.name}.jar" out="${build.extension}/private/${ant.project.name}.jar"/>
<externalclasses refid="build.classpath"/>
</obfuscate>