Ask early. Sometimes approval takes a long time. He/she may have to ask higher-ups.
Warm them up. One day mention how you wish you had better tools to do something or wish you knew more about something ("time management" maybe?). A few days later say that you found a class on the topic at LISA. (Intro to TM and Advanced TM are both being offered)
Talk about end-results, not technologies. "There's a class that will teach me how to automate installations" is much more understandable than "there's a class on Puppet". Find 3 classes or talks you want to attend. Explain them in terms of the results the boss will see. 2 is good, a 3rd is good as a backup. More than 3 sounds like you are trying too hard.
If they have a favorite sysadmin book, look to see if the author is speaking at the conference. If they respect the author, they'll be more open to sending you to the conference. If they like any of my books, tell them about my new class!
Tell them they can come too. With classes like "A Sysadmin's Guide to Navigating the Business World", "Workplace Presentations 101 for System Administrators" and "Team Efficiency" there is a lot for them to learn too!
Usenix LISA 2011 is Dec 4-9 in Boston. You can register any time, but you get a discount if you register by Nov 14. I look forward to seeing you there!