Side note. Yay! you have 237 posts. cool!
Anyways, The first step is to approach people. A lot of people have the best plans and intentions to ask people to sponsor them, but don't follow through.
Actually asking for sponsorship is the first step.
Once you do this, it is a hit or miss game. Some people may tell you straight out no. Some others may tell you to leave them some info. Others may tell you that it is not their call (more so in the retail business - may redirect you to corporate HQ or even a District manager.)
Have fliers to hand out. At team 237 we have a tri-fold brochure that we give out to companies looking for sponsorship.
If you are a 501c3 Non-Profit, make that clearly known to potential sponsors. This is very useful to them especially around tax time as they can write off donations to you.
Bottom line.. as our motto is on Team 237 - Fund-raising is a year round job for everyone. Don't just fund-raise a month or 6 weeks out of the year.
Also, don't count out the little fundraisers like car washes and bake sales. Although a dream sponsor who will pay for everything may be found just by approaching someone, it is very few and far between unfortunately.
Also, don't reject any donation. Any donation large or small should not be refused. If the local big retail store can only give you a hundred dollars or so even though they are a multi-million dollar store, don't forget that thank you note. (Another important thing.)
If they donate this year, and then you promote them somehow (press releases or whatever) they may give more next year.
Like I said.. Hit or Miss...