How can they have comercials or clips of next week's episode if it's going to be live?

They'll just cut to a commercial break like they would on another live show like SNL or American Idol or whatever, and they'll take a break in the studio.

30 Rock did two live shows recently if you're interested in knowing what they're like:

http://en.wikipedia.org/wiki/Live_Show
http://en.wikipedia.org/wiki/Live_from_Studio_6H

@EmoJoe: I think what the OP was asking was how can we be seeing clips of the upcoming live episode in commercials when the live episode hasn't aired yet.

My best guess is that they were the rehearsals or perhaps clips from previous episodes?

Thanks to both of you.

Yes, I was wondering how they could show clips of next week's episode when it hasn't aired yet. It probably was a rehearsal or something like that.

I think they're just now rehearsing it this week, but it's probably just clips from past episodes, or the new ones they've already filmed.

Oh really? That's weird, but yeah I'm guessing it's either rehearsal or maybe clips taped specifically for promo.