You don't have to do the mantra or visualizing while you are falling asleep.

When you are visualizing or repeating mantras, you are performing what is known as rehearsal. Rehearsal strengthens memories. There are two types of rehearsal. Repeating mantras and visualizing are the two different types.

Repeating mantras is an example of maintenance rehearsal. Maintenance rehearsal involves repeating something over and over without thinking about its meaning or connecting it to other information. Maintenance rehearsal is best for holding something in your short-term memory. However, your short term memory only lasts about 20-30 seconds, so once you want to remember is no longer in your short-term memory, it can only be remembered from your long-term memory. Because you are likely to be unconscious for more than this time before the dream begins, it is best to focus on making what you want to remember a long-term memory.

Visualizing is an example of elaborative rehearsal. Elaborative rehearsal involves thinking about the meaning of the memory being strengthened and involves connecting it to other information already stored in memory. Elaborative rehearsal is best for converting what you want to remember into a long-term memory. In the case of visualization, you are basically trying to learn something that you want to remember in the future. For example, if you missed a dream sign, you want to visualize yourself catching that dream sign and intend to catch that dream sign in the future just like you are visualizing. This way, next time you come across that dream sign in a dream, you are more likely to catch it due to what you learned from the visualization.

Mantras are helpful for keeping you from become distracted while you are visualizing and for those times where you are unconscious for less than 30 seconds before the dream starts. However, it is the visualizing and other types of elaborative rehearsal that is helpful for converting the memory you want into a long-term memory. So, once you are confident you have learned what you want to remember well enough, holding something in your short-term memory is not necessary so you don't have to do mantras or visualization anymore while going to sleep.