Don't use canned lines. Simple. Use your wit and make an observational joke/comment based on the situation.
I agree, but we have consider the timing of the thread. It was started in 2008 when canned game and c&f were still popular (though canned game was starting to decline).
I certainly agree that a random situational comment will be more genuine and work better. The randomness can make it very natural and smooth.
When you try to "recycle" or "reuse" a line, it's more tricky because your delivery may not be as good as the first time. For example, I would love to reuse my "You're soo sleeping on the couch tonight" line (from earlier in the thread), but it will probably come off a little choppy and not as smooth and funny as when I used to use it.
Notwithstanding all that, a canned line or two in your head as a backup plan in case you "draw a blank" is very helpful.
Even a not so smooth canned line can work better than freezing up and not saying anything.