Ewan,
I see that you use Salesforce.com for your CRM, and I find that the people to whom you must "prove marketing's worth" are likely using Salesforce.com vs Marketo.
Therefore we optimize SFDC campaigns - by using Marketo as the "back-end" workhorse, and putting the metircs in dashboards in Salesforce "front-end".
As Josh referenced, there are a lot of resources out there for specific metrics, but I'll mention a few classic ones. Our non-marketing colleagues care about things like "Marketing contribution" to pipeline and revenue. This is usually expressed as a % of total. They also care about "ROI" - the cost of those marketing campaigns compared with the results. Then you can talk about which campaigns are better than others. Etc - from there the # of possibilities/metrics are endless.
But I guess more importantly, you can show that if Marketing were to go away, it would have a fairly well defined (negative) impact on revenue.
Eventually you want to get to a place where Marketing comes up in planning for revenue by reversing your metrics: we want to have a revenue of X next year, therefore we need Y campaigns, which will yeld Z results, etc, etc.
Make sense?
Hope that helps!