http://elblogdepicodev.blogspot.com.es/2012/10/usar-variables-en-un-proceso-de-negocio.html

Variables.bpmn20.xml

<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:activiti="http://activiti.org/bpmn" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.omg.org/spec/BPMN/20100524/MODEL http://www.omg.org/spec/BPMN/2.0/20100501/BPMN20.xsd" targetNamespace="HelloWorldActiviti">

    <process id="variables" name="Variables" isExecutable="true">
        <startEvent id="start" />
        <sequenceFlow id="flow1" sourceRef="start" targetRef="descuentoTask" />

        <scriptTask id="descuentoTask" scriptFormat="groovy">
            <script>
                output.descuento = new java.math.BigDecimal('0')
                if (cliente.tipo == es.com.blogspot.elblogdepicodev.activiti.misc.Cliente.Tipo.VIP) {
                    output.descuento = new java.math.BigDecimal('10')
                }
            </script>
        </scriptTask>
        <sequenceFlow id="flow2" sourceRef="descuentoTask" targetRef="end" />

        <endEvent id="end" />
    </process>
</definitions>