The first biological factor that seems to be involved in long term memory is sleep. Much of long term memory is encoded and stored during sleep, especially during dreaming or Rapid Eye Movement sleep. Sleep deprivation impairs memory.
Age affects our ability to lay down new long term memories, and the speed at which we retrieve them, but not necessarily our long term recall.
Physical health, including exercise and healthy diet, may improve both long and shoprt term memory and delay the onset of Alzheimer's.
Various chemical substances can improve or weaken memory, with alcohol having a negative effect on memory.