The best bet is to aim for spring or autumn, when temperatures are mild enough that the city doesn't become claustrophobic (or smelly!). The famous carnival usually falls in February, as does Valentine's Day, both of which draw huge numbers of visitors. We visited at the end of February (just after these events) and found the city to be relatively quiet, creating a stunning, peaceful mood. Winter can be incredibly atmospheric (particularly if it snows), but it can also bring acqua alta (floods caused by high tides). If your visit does coincide with one, don't panic: they usually only last a few hours and the Venetians are well prepared (often with spare wellies at the ready!).
Note: flight, boat, train and bus timetables change constantly, and airlines come and go, so please do not rely solely on this information for your travel planning. Check with relevant companies, or a flight search engine like Skyscanner, first.
Venice is a world-famous centre for tourism so flights are available in abundance. The main thing to consider is whether you want to fly into Venice Marco Polo (the main airport for the city) or Treviso, which is a 40-minute bus ride inland.
From the UK:
Flights from London take a approximately 2 hours and 30 minutes.
Ryanair flies from London Stansted, East Midlands, Leeds and Bristol to Treviso.
easyJet flies from selected London airports and Manchester to Venice Marco Polo.
British Airways flies from London Gatwick and Heathrow to Venice Marco Polo.
There are various charter and low-cost flights into Marco Polo and Treviso. In addition to national airlines, Ryanair and easyJet offer frequent services from various European cities.
Venice is easily reached with the Eurostar from London (changing in Paris), most Italian cities and the major European hubs. Trains arrive into Santa Lucia train station, from which there are frequent vaporetto services to the rest of the city. See the excellent Seat 61 for details.
On arrival in Marco Polo airport, you can take a bus or taxi to the Piazzale Roma, or travel by boat (water bus or private water taxi) to various locations in Venice. Tickets can be purchased in the airport, or booked in advance. We like to take the Alilaguna boat - much more atmospheric than the bus / vaporetto combo, though a shade slower. If you're flying into Treviso airport, there are frequent buses to the Piazzale Roma (40 minutes). Buses are coordinated with flight times and tickets can be purchased in the airport terminal.
Water taxis, vaporetti and gondolas
Buy a 24-hour / 72-hour vaporetto pass to save shelling out for each journey (vaporetti are the water buses of Venice). Beware of fake water taxis that charge over the odds and drop you in the wrong places. Don’t neglect the gondola traghetto (gondola ferry) across the Grand Canal - there are 4 crossing points, which can save a clot of time, and are a quick and (much cheaper) way of experiencing a gondola.
Take a suitcase with wheels to get between vaporetto stop and hotel - but not so heavy that you can’t lift it over the stepped bridges. In winter - when periods of acqua alta (high waters) are likely - take wellies, and bear in mind that wheeled cases and pushchairs may be hard work.