Avoid boiling the pasta for too long to avoid tearing. Sometimes I boil it a minute or two less than the time suggested to make sure it doesn't tear. Make sure to be gentle when lifting them out of the boiling water as they do tear easily when they are moved around. However, even if manicotti tears a bit, it shouldn't matter. You don't really see the tear and the filling should stayed stuffed well even if it tears a bit.