There is no trick when it comes to those any more than the rest of the coin choices. It is the same as picking the ICO, you will never know until the last minute and that is how it is always going to be. There are those that push and make you think that it is going to happen and then when the moment comes, bam, they are gone and there is nothing that you can do, at least with the air drop you have likely not invested anything but time.