The glans and the rest of the corpus spongiosum is not supposed to get fully hard. Remember, the urethra runs through it... if it got super hard, it would compress the urethra and make ejaculation difficult.

That being said, I have noticed that squeezing the same muscles used in Kegel exercises does tend to pump blood into the corpus spongiosum and fill up the glans... it provides an overall harder erection, but ejaculation force/distance suffers...