For some entrepreneurs, prioritizing yourself can be one of the biggest challenges. Here's how to learn to say no and set boundaries.

Helping others, sharing, being kind: These are the sorts of values that have been instilled in most of us since kindergarten. And they're great values that make us better spouses, parents, neighbors, and citizens.

But sometimes, when you're an entrepreneur, being nice to others means shortchanging yourself and your business.

As Renée Warren, co-founder of Onboardly, reflected when asked what she wished she'd known starting out as an entrepreneur, the value of your time is one of the hardest things for some founders to learn. And safeguarding this resource requires saying no and being (constructively) selfish.

"It took me years to finally start saying no to things that would take me away from what really needed my attention," she says. "Time is the most valuable thing you have. Make sure you invest it wisely."

No one suggests being rude or truly self-centered, but according to time coach Elizabeth Grace Saunders, always trying to please others gets not only Warren but also a lot of busy professionals into trouble.

"In some jobs, immediate responsiveness comes with the territory (just think of fire fighters)," Saunders wrote on Lifehacker. "In others, a quick reply is preferable, such as with customer service reps or publicists. But in many other work situations, this cycle of responsiveness leads to neglect of the most important activities. Either they don't happen at all, or you end up filling your nights and weekends doing your 'real' work. I've worked with clients on six different continents who come to me feeling like victims of their circumstances."

So what can be done about this lack of boundaries? The short answer, which everyone already knows, is to say no more often. But if that were as easy as it sounds, so many of us wouldn't be struggling to manage our time.

Handily, Peter Bregman, writing on the HBR Blog Network, offers a mental toolkit of nine practices to help oversolicitous entrepreneurs get better at saying no and avoid both exhaustion and calendar chaos. Among them:

Be as resolute as they are pushy. Some people don't give up easily. That's their prerogative. But... give yourself permission to be just as pushy as they are. They'll respect you for it. You can make light of it if you want ("I know you don't give up easily--but neither do I. I'm getting better at saying no").

Establish a preemptive no. We all have certain people in our lives who tend to make repeated, sometimes burdensome requests of us. In those cases, it's better to say no before the request even comes in. Let that person know that you're hyperfocused on a couple of things in your life and trying to reduce your obligations in all other areas. If it's your boss who tends to make the requests, agree up front with her about where you should be spending your time. Then, when the requests come in, you can refer to your earlier conversation.

Be prepared to miss out. Some of us have a hard time saying no because we hate to miss an opportunity. And saying no always leads to a missed opportunity. But it's not just a missed opportunity; it's a tradeoff. Remind yourself that when you're saying no to the request, you are simultaneously saying yes to something you value more than the request. Both are opportunities. You're just choosing one over the other.

The remaining six suggestions are just as helpful, so check them out at HBR Blog Network. Saunders also has useful tips. She breaks down those who struggle with saying no into three categories--those with unrealistically high expectations for themselves, extremely service-focused people, and folks who are a bit delegation challenged--and offers advice for each.

Do you struggle with saying no? How do you cope with the problem?