BDNF is a chemical released into the brain which helps increase Neurogenesis in the Adult Brain and increase the complexity of older neurons.
BDNF increases a neuron's excitatory effects
Ways of increasing BDNF
Exercise
Can increase the concentration of BDNF in the hippocampus
This is of rat brain, left is rats with a hamster wheel to exercise with.
Diet
Limiting the amount of food you intake can also affect the amount of BDNF.
So does limiting you food intake for a period of time (even though your total amount of the week is the same)
What you eat also effects things
Eat more low-fat complex carbs.
Learning
If you learn things, your brain grows.