"It is best to have the body at rest and the mind at peace and cheerful when one starts yoga. In the absence of such a state, it is advisable to lie in Shavasana or sit in any of the meditation postures till the body and mind are refreshed and quieted and only then do one's recommended yoga."
His advice is important to hear and apply because too often we're too hard on ourselves. Already exhausted or stressed, we say "now I have to do my yoga practice." Consider the wisdom of delaying the actual practice by preceeding it with a nice quiet shavasana. Then proceed!