Plug for Brennan's plug: This book is amazing. I've increased rates by +$30/hr with the advice in this book. If you send invoices to clients BUY THIS BOOK.
If you provide value, and you're good at what you do, why not ask? If it's because you can't handle being told no once in awhile, maybe Rejection Therapy[1] is the best way to go. The game can be applied to all aspects of life, not just getting higher hourly rates.