I have just had an email from Ellen Maria Wright, who is about to work in Lithuania. This is the question that prompted her to write:
I think the main reason why teachers find it so difficult to identify appropriate forms to focus on is that they see the aim of form focus as providing learners with the ability to use new forms more or less instantly. In fact this never happens.
The purpose of a focus on form is to highlight new forms for learners and make them aware of the long term need to master these forms. So it's not a matter of a blitz on, for example, the passive, in the hope that learners will assimilate this new form as a result. It's more a matter of making them aware of the passive and giving them some insights into its use. As a result of this they will be more aware of the passive in their future reading and listening and so kmore likely to begin to use it.
If you take this view then you don't need to find a text which is massively packed with examples of the target form(s). You just need one or two examples. You can then supplement these with examples from other texts your learners are familiar with and you can look mout for supplementary example in future texts.
I think the imprtant step is to get away from the belief that we can control what learners will assimilate and the temptation to aim at total mastery.