I have been very happy with an easy, free, browser-based screen sharing facility called Join.Me. That is also the web address: join dot me (not dot com).
I often “dig into the numbers” with my CEO executive coaching clients, sometimes with the help of my free Excel templates (see them here). This tool allows us to see and work on the same screen together though we are miles apart.
Join.Me is so simple, with no software to download or install and no firewall or security issues, that even top executives can use it on the fly.