0) Rule zero. Secure the venue and set the date (Done). Give yourself enough time in advance. Start early (June? 61 days? start today!)
1) Come up with different levels of sponsorship: Gold, Silver, Bronze? $100/$300/$500 (or whatever) Each has a different price and each comes with its own bells and whistles: Logo placements on program, website, banner hangings in the hall, MC call outs, a display table, different forms of recognition to differentiate. Make it fun.
2) Easy to produce marketing tools. Make a sponsorship kit. Talk about the event, the attendees, include any media clips or links to same. This Kit could be a fact sheet, a brochure, a website, all of the above. Something to use when you talk to people who can give you money
3) Social media tools. Twitter, facebook, cheap and easy wasy to get the word out and increase the event profile. If you have video chops, do something with footage and post on youtube. Link madly. You got these, so good work. Leverage it. Put your kit on a tab on the website.
4) you have a list of 300 people. Good work. Call them, email them, let them sponsorship opportunities are available. Some of those people know businesses, others are businesspeople.
6) Industry. What games are you running? Contact the publishers, see what they can do. It could be money, it could be free schwag. Something. Also local hobby shops ~ game vendors. Make a list and contact them all. Send them some press before you do that. Cold calls are tough, warm calls better. ASKING is the important thing.
7)Supporting vendors. 300 people is a lot of hungry people. Easily an opportunity for a few food and drinks vendors. Have a sponsor underwrite a snack: "morning cookies and coffee brought to you by company x". Do you have an awards banquet? That could be sponsored too. What about event t-shirts and cool souvenirs? Do you have a raffle? Lots of room for fun here.
That's just getting started. When you think about it, there's a lot of ways to monetise a con.
Oh and you might have to pay someone to execute all this. Give them 20% of all sales as an incentive.