Make sure you give them plenty of input and plenty of time to practice the phrases you want them to learn and use. Sometimes it can be a case of students just not being comfortable with it, even though it might just be a couple new words, or a phrase they should have learned three years ago.
Authenticity helps, too. Information gap activities work well, where they have to actually find out pieces of information about their partner(s) and classmates. If it's just reading from a book
Can Zeeto ride a bike?
No, he can't.
Can Minsu swim?
Yes, she can.
over and over and over for ten minutes (as the books sometimes suggest as "speaking" practice), there's really no point and it's definitely not an authentic speaking activity.
For something like can and can't, to use an example brought up by Scott, the "Find someone who" activity works well because it requires them to talk to a variety of students, actually get information they don't have, and authentically use the language.
http://waygook.org/index.php/topic,709.0.htmlhttp://waygook.org/index.php/topic,306.0.htmlJust one possible idea out of many.