First of all: You are endorsing the company. They are not endorsing you.
Disclaimer: I am not an expert on this issue, this is just a collaboration of posts from previous endorsement threads to help prevent more unneeded threads.
Thanks to these Users for their infinite wisdom:
hasbeen
Spoiledgrape
Woodchuck
Fretlessboy
Smperry
Rusty chainsaw
www.getsigned.com (the website) Some FAQ’s What are the different levels of endorsement?
I broke it down to these types:
Average Joe- You just endorse the product without anything in return from the company
Artist Discount- You endorse the product and get a discount on all that companies products.
Pro- You endorse their product and get free gear from that company.
Super Pro- You endorse the company and get a signature instrument. [rare]
Do you approach the company or do they approach you?
Most of the time it will be you going after the company. Unless you are a very well known artist, chances are you won’t have all the big names chasing you down.
What are some ways to add some beef to your resume/press kit?
- Tour Relentlessly
- Make lots of connections with people and have friends with “benefits”
- Play with an already accomplished Artist/Band
- Do lots of work in the studio
- Make a website, myspace, or a youtube channel, etc. to show your skills.
Press Kit? http://www.getsigned.com/presskits.html
A press kit should have these things in them:
-Pictures of you on stage, and preferably showing you with the companies product.
-A Biography of your musical career. Be to the point, with little fluff.
-Any press coverage/newspaper clippings/online articles about you our your band.
-A CD demonstrating your chops, and some tracks from your band.
-A detailed gig sheet from the venues you’ve played at
How Soon until I get my free stuff?
Instrument endorsement deals are usually different than any other, since the musician is their customer, it is very rare to get a "freebie." Artist discount is usually at about 25%-70% depending on the deal, and how important you are to the company.
What is a good model for getting an endorsement deal?
"Hi, I'm [your manager or A&R] I manage [you or your band]. "My client played your rig/instrument at the last [gig, tour, etc.]. He/She loved it. He's going on tour with [Band or Artist]. You can log on to [the band you‘re touring with] and see there tour schedule. You can also go to my artist's site at [your site]. If you support us we will support you.”
Make sure you also include:
* what your objective is.
* what promotional consideration the company will get in return.
* what your qualifications are.
Thanks for reading.
I Realize this will definitely not answer all questions about endorsement deals. I just threw this together as a short guide. All comments and questions are welcome. As I said earlier, I don’t know much about this, but there are plenty of other members on this site that can help you out. 
Now get out there and play! 
