Comfortable walking shoes are a must. You will spend most of your time walking, and there are lots of bridges with stairs. Flipflops are not practical for this, and locals only wear them at the beach, never in town. High heels and platforms are an invitation to twisting your ankle unless you're accustomed to walking on cobblestones (though the paving in Venice is generally more even than cobblestones).
In high season, you'll want light, cool clothes.
Although it's often said -- and there's some truth to it -- that Italians in general and Venetians in particular look down their noses at tourists wearing shorts, they're probably the most comfortable choice from June well into September. Very few churches these days will turn you away for wearing shorts (as long as they're not ultra short and tight); biking shorts, bathing suits, or leggings without a skirt are not advisable. Many churches require that women's shoulders be covered, so carry a light shawl in your bag if you're wearing a sleeveless top.
Venice does get cold in the winter. From December through January, be prepared for temperatures as low as 4 C / 40 F.
Rain is possible at any time of year, so a travel umbrella or a raincoat is useful.