Know their schedules, and plan for a time when they will be away for a least a few hours.
See their pleasure as your goal line.
Do some research and see what you might be missing out on.
Another trick is to blow gently on her clitoris before making contact with your tongue, which can increase arousal.
I don't recall which gave out first -my penis or my right arm.
Engle suggests touching it through the clitoral hood; another idea is to perform oral sex with a thin piece of fabric, like underwear, between your tongue and your partner's body.
So splurge a little bit.
I've only done it a couple of times, but I can do it.