There's another very simple answer which can work for some simple graphs.
Just add a call to guide_legend() into your graph.
ggplot(...) + ... + guide_legend(title="my awesome title")
As shown in the very nice ggplot docs.
If that doesn't work, you can more precisely set your guide parameters with a call to guides:
ggplot(...) + ... + guides(fill=guide_legend("my awesome title"))
You can also vary the shape/color/size by specifying these parameters for your call to guides
as well.