Learning from someone else's experience is invaluable. Mentors can help you break into new fields and navigate unchartered waters. Not only they can help you become a better version of yourself, but they also advocate for your success.
Every mentorship relationship starts by asking for advice. But it can be hard to get advice from someone who doesn’t know you yet. Starting that conversation requires courage. Not only you have to admit you need help, but there’s still a possibility your request will be rejected.
People, after all, are busy - or at the very least they think they are - and it’s easy for busy people to say “no”.
In this piece, I won't get into details of how to ask for mentoring, what medium to use, or what to say. I'll focus instead on two tenets that, if you follow them, will greatly diminish the chances of hearing a "no".
Always assume they are busy
In general, people like helping out. It makes them feel appreciated. That appreciation vanishes the moment they feel you’re taking their time for granted.
Busy people want you to understand that they are busy. You are reaching out. You are asking people for their time, so respect that time. You should work around their schedule, meet them at their location, use their timezone. In other words, they want you to acknowledge how busy they are.
But it doesn’t stop there. Save them time being clear on what you need help with (you're asking for advice, remember?). Instead of writing a long-winded email with no point or babbling about things that ultimately won't matter, be specific in what you want.
Help them decide how to answer you by including a call to action. If there's no call to action, the burden of deciding when and how to respond will fall onto the other person. Always remember that the easier you make it for them to counsel you, the smaller the chance of hearing a “no”.
The same goes if you don't hear back from them: follow-up once, but don’t hound them. A check-in after your initial contact is fine, but otherwise assume they don't have the time.

Be mentorable
Show your mentor that you mean business and that you are serious about getting better. It’s important for them to know that you got your hands dirty before asking for help? Share any evidence of your progress so far.
Then, value the feedback you get by acting on it. It doesn’t matter if you take excessive notes and hang on to their every word. If you never act on the advice given, it’s unlikely that your mentor will help you again.
Finally, provide updates on your progress. Let them know how far you’re going with their help!
One common piece of advice is that you should offer value to the person you're asking advice to first. I disagree. The act of mentoring is an investment in itself. Someone who’s mentorable can turn advice into growth. This drive is admirable and mentors treasure to connect with people like that.

What if you have ulterior motives?
There's one caveat to the tenets above: when the reason you want a mentor is not the mentoring itself. For example, if your ultimate goal is to work under that person.
In this case, here's what you should not do: ask to work for them. It usually backfires, as people can smell these opportunistic approaches from a mile away!
Focus on being mentored instead. Ask how can you break into the field or, if you’re already in said field, what do you need to learn to improve. By turning their advice into growth and showing them evidence you position yourself as a potential contender. That way, if an opportunity comes, they will consider you favorably.