I know you're probably amazing at whatever it is you do but I'm sure there are areas that are outside your depth of knowledge - areas where maybe you aren't so amazing. No problem, that's why you have experts as friends, colleagues, and mentors. If you have the budget, hire them. If you don't have a budget, offer an exchange, or buy them dinner and drinks. I've picked many brains over a beer.
Consulting experts will get you to solutions faster and shorten your conversations with clients. That's more upside for you if you're working on a project or flat rate. If you work hourly it means moving to your next project sooner.
I'm consistently surprised how often people are willing to offer advice or insight around their area of expertise. Don't abuse this. Show appreciation, give them credit when possible and just be gracious. Show willingness to give back. The interviews on ools not Rules are an example of this. Some of the people are my friends and colleagues but some of them are people I've never met, or barely know, but whose work I admire. For a little bit of their time I try to spread the word about their talent. It's not much, maybe, but it's genuine.
Where do you lack expertise? Now, which of your friends and colleagues have the skills you lack? Reach out to them. Don't worry about being a pain in the ass, you'll return the favor at some point and that's cheaper than going to school or learning the hard way.