Avoid the main routes
There are basically two main tourist routes in Venice, both conncecting the train station Santa Lucia to Piazza San Marco. The first one is called Strada Nuova, and is a broader road that can be taken by going left and then straight ahead after having exited the station. The Strada Nuova leads you to San Marco almost directly (in spite of several twists and turns), with fewer bridge crossings than any other route. The second one starts from the Scalzi Bridge (Ponte degli Scalzi), which is right in front of the station, proceeds throught the Sestiere Santa Croce and San Polo's, crosses the Rialto Bridge and again leads to San Marco.
Be aware that almost 80% of the tourists can be found along these routes and in the area surrounding San Marco. There's a huge part of Venice that rarely gets visited by tourists, for example the Castello Sestriere and the area around the Arsenale, or the Northern parts of Cannaregio. These are the places where you can see the 'real' Venice (if there is one), and they are not short of charm at all. Furthermore, most things tend to be much cheaper in those areas, including restaurants, shops, bars and lodging.
Beware of tourist restaurants
While some restaurants are indeed honest and, even if the prices are still expensive, you can eat well, many of them are tourist traps and the quality of the food is not in line with the price at all. The trick is, most tourists go to Venice once in their life, and if they decide not to come back to the restaurant, that's no problem.
One of the worst is probably pizza: while being one of the best and cheapest italian food specialties (and one not worth tasting outside Italy's borders), it is very rare to find a good pizzeria in Venice.
So, before choosing a restaurant, have a look at what other people are eating there, or ask for advice to people who have no special interest in giving it to you, that is people not directly involved with the tourist industry.