You need to
1) Persuade the brand / shop that you are an opinion leader in the appropriate market segment,
Usually by actions/reputation rather than persuasion in their office. (Self fulfilling prophecy - if they've heard of you, they'll believe you - if they havn't heard of you, you probably aren't

)
2) Persuade them that you will be a good brand ambassador for them. Nothing breaks credibility (yours and your brands) worse than a sponsored athlete who doesn't really believe in the kit but likes free stuff, or an athlete who is sponsored by a different brand every season.
Think of it as a marketing contract like any other.
Lastly, don't kid yourself that you're going to make a fortune and retire young. It just makes a few more climbing trips a little more accessable.