Sous vide literally means "under vacuum," but the difference between a true airtight seal compared to a mostly airtight seal is less than you might think. The gain from cooking sous vide is much more about temperature control, rather than the air-tight seal of its packaging. Even when something is sealed perfectly, the food you are cooking is going to constrict from the heat and there will be some extra room in the bag. Ideally, we would all have the money for a VacMaster Portable Chamber Vacuum for perfect seals all the time, but there are plenty of other ways to go about it.
In the gif above, Douglas Baldwin seals a chicken breast in a normal Ziploc bag through water displacement. As the water pushes down on the bag, air comes out and the result is a surprisingly effective imitation of a vacuum seal.
Ziploc bags are great for sous vide and will meet the needs of most people, but according to Nathan Myhrvold of Modernist Cuisine, Ziploc bags are not suitable for long term quick-chilling. One of the benefits of sous vide is that if chilled properly, food can be safely stored for up to a year in the freezer and reheated in under an hour with minimal loss of flavor or texture. Over a long period of time, even the Ziploc freezer bags will allow trace amounts of air into the bag that can create some degradation in the flavor and possibly the development of food pathogens.
A slightly more advanced version of this is to use Ziploc's specially made Ziploc Vacuum pump system. The vacuum bags are more durable/reusable than traditional Ziploc bags and have a valve built into them, but are about twice the price per bag. Instead of using water to force the air out, you can use a handheld Ziploc pump to suck air out of the bag through the valve. An important drawback of this method is that you cannot use meats with liquid marinades (unless you freeze them), but I did find this to have a tighter seal than Ziplocs that just used water displacement.
The less than perfect vacuum seal can sometimes create pockets of air that floats your bags to the top and ruin the temperature control. As an insurance policy, normal binder clips are great to keep the plastic rigid enough to stay underwater.
For those who are worried about the environmental impact of cooking with so many plastic bags, the dishwasher-safe silicone bags from Lekue are a great option. Due to the thicker plastic of the bag, the seal is not as tight as a ziploc, but it will serve the needs of most cooking situations. It is only one liter in size, so you might need more than one for large cooks.
For those who are looking for a tighter seal, the Foodsaver edge sealer might be what you are looking for. While the machine costs about $80, you will also need to factor in the costs of the bags, which cost about $9 for a 8-inch by 20 foot roll or 11-inch by 16 foot roll. That being said, the FoodSaver offers a couple of important benefits.
With an edge sealing system like the FoodSaver, air is pumped out of the bag through an open end that is later sealed. In combination with the sturdy 5-layer polyethylene bags, the tigher seal creates a very durable enclosure for food. It makes it possible to buy food in bulk, store it in your freezer for months and cook food sous vide without needing to wait for food to thaw. This also applies to cooked food, which can be cooked on large scale and then safely stored in your refridgerator or freezer.
The only downside with the FoodSaver is that liquid marinades or sauces can get sucked into the vacuum machine and prevent a seal from forming. Vacuum bags have to be dry where the machine applies its seal, so do your best to keep the outer edges of the bag as dry as possible.
To get around this, you can hang your bag off the edge of your counter and use gravity to hold the liquid down. You can simply stop the machine just before it starts sucking up liquid. Also, you can freeze your marinades beforehand. Some like to freeze it with their food and others like to freeze their marinades separately.