After experiencing it personally and going through various forums, here is what works for me.
- Try undressing or at least open the clothes (easier for onesies). odds of working low-medium.
- Take a semi-wet piece of paper towel and gently dab on baby's face. Baby should respond to the coldness of touch and get a little alert. Odds of working: high
- Slowly blow some air on baby's face. Warning : be very gentle. I blew little hard the first time, she woke up little scared and threw up. But done right and baby should be alert. Odds of working: high
Again this just my experience and not any recommendation or suggestion.