Centering content in sidebar

Hi,

I’ve been trying to center the content of my right sidebar, i.e. I want the ads I’m running to be centered, rather than up against the left border of the sidebar. I’m pretty sure it’s just a simple command, but after repeated attempts I’ve not been able to manage it. Can someone help?

(You can see what I mean here: http://www.newsite.aldasigmunds.com, under ‘sponsors’.)

Many thanks!
Alda