If you really want to show people you are serious, you have to show them what they wil get. Make the magazine (issue 1) with the ad spaces open (i.e. "See your ad here"). Go to some local gameshops (not the big ones, 'cause they wont care) and try to sell some ad space. Tell them this is the magazine. This many people will see it every month, and this is what it will cost you for an ad.