Bikram says, "It's not what you do, but how you do." So don't let the mind wander during practice. Don't be too aggressive or impatient, either. The only time you will hurt yourself (create a sprain or pull) is if you overuse your strength or do postures mindlessly. Think of alignment. Bio-mechanically you are very safe if you do postures in alignment.   

Muscle imbalance creates a lot of problems. Check to make sure there is good alignment with the left and right side of the quads.  Balance with the foot in alignment and the weight in alignment. Remember, the body follows the eyes. Be careful to do the postures correctly, because you are creating an imprint that is hard to undo.

Breath is the only way to affect the involuntary systems. Oxygen deprivation is a major cause of spasms, other muscle pains, and sciatica. Use breath to break through fear of pain-- sigh a relaxed sigh. Deep diaphragmatic breathing is best when dealing with lots of resistance. Send the breath there--breathe through the nose.

And finally, honor your boundaries, but expect them to expand. Have faith!

When Menstruating? 

It is perfectly safe to do Bikram's yoga when menstruating. Inverted postures are normally the postures contraindicated for menstruation. But there are no inverted postures in Bikram's yoga.  

So in fact, Bikram's yoga is very good for toning the reproductive system as it directly affects the reproductive organs and the endocrine glands--pituitary, pineal and thyroid gland in particular. The women's cycle becomes regular; complaints of irregularity and PMS decrease.