HI DEP,
I may be a bit late in responding to this post. But I hope my inputs will be useful.
First, understand that any company will not sponsor an event unless they know about your college and its reputation in the market. So you must carry all the relevant college brochures with you. If possible, also carry references from college professors.
Second, you have to portray a distinct value to the sponsorer. Every company will have different motives and they will not put money unless they find a benefit out of it. So as Kartik said, its good to have a documented portfolio of your events. Its also advisable that you carry the past data of your college functions, like how many competitions were organised, how many students participated and what was the visibility for past sponsorers.
Third, Its not compulsory, but helpful to have an existing contact in the company. If you know some one, use him/her.
Fourth, HR is the wrong department to contact. Agreed that they will have all the time in the world to listen to you. But may not give you sponserships. You should instead target the
PR or the corporate communications department. These are the guys responsible for sponsorships and advertisements.
Fifth and most important, dont lose hope. If you don't quit, don't cheat, and don't run home when trouble arrives, you can only win.
I think every thing else is covered. Best of luck!