Can people improve their memory by training themselves to recall previously forgotten memories?
Memory is not fixed - it is very plastic.  If you look at Amnesia, there are many different causes and many different ways it can manifest itself. It can also be anywhere between mild and severe.  There are methods which can be used to try and prevent further memory loss - by abstaining from alcohol when it is caused by alcohol abuse such as with Korsakoff's syndrome, by using diaries, calendars, wall planners etc. But just like with Alzheimer's Disease there is no effective (and ethical treatment) for memory loss.

Which brings me onto the ethics of memory recall therapy or training.  If you look at studies into memory and PTSD (Post Traumatic Stress Disorder) - also found in category F43.1 in ICD-10 - methods used to try and help sufferers to remember the traumatic events for this example, have been found through research to cause False Memory Syndrome.  Although as with many pieces of research, there are opponents to the idea.

From what I know, and believe from looking at research, once memory is lost, it is lost for good.  You may recall parts of the memory through various ways but to try and recall the whole memory can lead to a change in what actually happened as @Arnon pointed out in his comment.