If you want to be able to remember the content of your trances, Aleister Crowley has methods for this in Book One ("Mysticism") of "Magick: Book 4".
His method is designed specifically for Mantra and Raja Yoga, which are both forms of concentrative meditation. Trance is a form of dispersive meditation. Dharana is an aggressive method of "holding on" while trance is a passive method of "letting go" (with some concentrative preliminaries, if you intend a specific sort of trance).
Crowley recommends keeping a notebook and pencil at your side while concentrating. This enables you to keep record of your errors and progress, to compare with your general advancement recorded in the Magickal Diary.
In the case of trance, start off with short trances, as short as you can reliably induce. If you know it takes you five to thirty minutes to enter into a trance, set the alarm clock for thirty minutes, and record in the notebook (a clipboard would be even better) exactly what you recall of your experience. If you recall nothing at all, record that. But record it. By remaining intent, this will gradually affect the nature of the impressions that you receive during trance, so that they translate more easily into ordinary consciousness.
This method also works for cultivating the lucidity of dreams, and eventually achieving astral projection.
The goal of trance should be to incorporate the senses accessible through trance into ordinary consciousness. If you are interacting with the astral/spirit/dream... world during trance but cannot remember what took place, this offers you no intentional benefit. This sort of trance might be compared to blacking out during a courtroom trial, or it might be nothing more important than waking up on New Year's Day wondering if you've cheated on your lover. We've sent monkeys into space too.