Things of the top of my head:
When and how often do you update? Especially important if you intent to sell adspace by the day.
What kind of things will be in your comic, a rating if you will. Fuzzy bear and Honey bunny's adventures in funland might make an advertiser think that your comic is all ages, which in turn might make said advertiser a bit angry if wednesdays strip features an orgie and/or massmurder.
Place and type of ads, will they are horizontal or skyscraper ads. Above, below or besides the strip.
If you have a substantial audience it might be a good idea to ask them to answer a few questions to determine what market segment they belong to as well as their habits of spending.