Social Policies: Liberty gives you a free MoV from Collective Rule, and you can pick another MoV when you get a free Great Person from finishing the tree. Commerce is also very good for MoV generation, as Entrepreneurship increases the rate of MoV generation, and finishing the tree allows you to purchase MoV with faith. I'm not sure which Ideology is best, but Freedom has several policies to increase GP generation and reduce the food and unhappiness generated by specialists, so you can more easily fill all of your merchant specialist slots.
Buildings and Wonders: Obviously, prioritize markets, banks, and such, so you have to merchant specialist slots. Anything that boosts GP generation is good too, such as a Garden and the National Epic. If you can't spare the people to fill those specialist slots, then make sure you get food-producing buildings like the Granary and Water Mill up quickly; the Hanging Gardens are great, too, if you at least open Tradition. Try to build the Colossus - you'll get two extra trade routes as Venice, rather than just one, and it gives +1 MoV point per turn. Keep an eye out for other wonders which give you MoV points, like Big Ben, Machu Picchu, and the Eiffel Tower.
Techs: Basically, anything that gets you access to any of the above buildings or wonders should be a priority.
Hope that helps; I've only played as Venice once so far, and I only made two city-states puppets, since I decided beforehand to try for a diplomatic victory. (Twenty city-states to start, puppeted the two militaristic ones, sent trade routes to the other eighteen and used Freedom's Treaty Organization policy to rack up absurd influence levels.)