How do I increase virya from the five strengths?
The goal or the focus should not be to develop virya, effort, energy.
The focus should be to develop mindfulness because mindfulness is the balancing faculty.
So you shouldn't ever worry about any one of the other four.
As you develop mindfulness, the five saddha, confidence, effort, concentration and wisdom will become balanced.
Morality, concentration, wisdom will be created and the Eightfold Noble Path will be created and enlightenment will arise based on mindfulness.
Virya should be balanced with samadhi, which is concentration.
If you have too much concentration, you'll become drowsy.
If that's the case, then you should focus on the drowsiness and be mindful of it.
Once you're mindful of it, you'll clear up the drowsiness and you'll find that your virya is increased.