I DID write a really long detailed answer to this.. And then the forum told me I wasn't logged in and I lost all I wrote X(
So here's a few quick things.. From someone who has a fair few endorsments (Not bragging... just stating a bit of a position to at least make a knowledgeable comment on this)
About 60% of my endorsements, I went after them first, the other 40% approached me.
Here are the main things I found that are important if you are seeking endorsement etc.
1. Dedication to the brand. In my opinion, I think endorsement whoring is horrible. Why bother going for 1 brand, which you generally HAVE to use at all times, if you don't TRULY enjoy what that company offers? My Jackson and MESA endorsements only came about because I truly do love both brands exclusively since I started guitar. So confidence/ love for the instrument/ brand etc...
2. Communication. Even if you're dedicated, and you love the brand etc.. If you can't string a sentence together when dealing with these companies, they won't care. This IS A BUSINESS after all. You've stopped being seen as Johhny someguy.. And are now THIS PERSON, FROM THAT BAND. And so you are an individual in a unit/ business etc... So act like it. These are generally professionals who expect to be spoken to as such. So yes, try put on the big boy voice when dealing with these people and show them you're not some illiterate child.. Sounds harsh, but I've heard enough/ seen enough horror stories occur.
3. How marketable are you???? At the end of the day, no matter how well spoken and dedicated you are, if they don't see you bringing in people as customers of the brand, then they still won't care. It's a business and they want a profit. How is signing you/ supporting you, in their best interest? In the endorsements I searched out, I had to prove I was going to be a good poster boy essentially and that my band had the following for it. For the OTHERS that approached me, it was BECAUSE they saw we had the potential to further sales and reputation of the brand.
That's really all there is to it. Well.. Easier said than done of course. But it really is common sense
Hope that helps mate.