Design product placement space ingame i.e. billboards, posters, food/drink items etc and then sell the rights to brand those items for use ingame -- product placement in movies is an example of similar -- what you would need to establish though is whether you are able to capture sufficient market, demographics etc to make the advertisers consider the investment a worthwhile return. Good luck - the odds are against you but not insurmountable.
Edit: Take note of what Promit said as you will have that battle on your hands as well.