We have two types of temporal memory: long-term and short-term. Clearly if you want to remember something for a long time, you should do whatever you can to shift the idea into long-term memory. There are many ways of making memories stick, but one factor to bear in mind is that the trace theory of memory suggests that most memories tend to fade over time and this is certainly what we all seem to experience.
When we encounter something new, we will forget a large proportion of the detail within the first twenty-four hours. Over the next few days, weeks and months we will forget even more although the rate at which we forget will drop off. Eventually, there will come a point when we only remember a small amount of the initial detail, but that small amount will be quite resilient. It is unlikely that we will forget any more.
To combat this fall off in recall, one thing you can do is to review information regularly in your mind. If you review the experience immediately after you have it, the following day, and then schedule a series of further reviews at increasing intervals - say after a week, then a month, then three months and six months, this will help fix all the details in your long-term memory and you will be unlikely to forget it any more.