If you're going to order wine in a restaurant, it will help to understand how restaurants sell wine. The better you understand the restaurant's perspective on wine, presenting wine, and selling wine, the more informed your wine choices will be. Ordering wine in a restaurant really is different, but it's still wine that you're buying so everything else you've learned about wine will help a lot as well.
How Restaurants Sell Wine
Buying wine in restaurants means buying wine from a restaurant's wine list. The wine list is usually organized by type (red, white, sparkling, fortified/dessert) then by varietal (Chardonnay, Merlot) and perhaps by region (Australian, French). In most restaurants, the wines are presented in ascending price, but even the most inexpensive wines that come first are far more expensive than what you would pay in even a fancy wine shop.
Wine lists present wines by name and vintage, but it's rare for any genuinely helpful information to be there. Name and vintage is informative to only a few experts; the rest of us need to know something about what sorts of flavor this particular wine has. If you're really lucky the restaurant has a sommelier you can ask for more information. If not, perhaps your waiter knows, but most just don’t have time to learn everything about the wines the restaurant is selling. In that case, you're on your own.
Avoid the House Wine
Most restaurants with more than marginal wine list will probably have a "house wine" in addition to all the branded wines they offer. Don't assume that this is wine specially created by or for the house, or even that this is wine which the house has specially selected for guests. Usually this is a cheap wine which the restaurant can sell for moderate prices and still make huge profits on.
You'll be fortunate if the wine is just average, though one exception to this generalization are restaurants located in and around wine growing regions. There, ordering the house wine can be a good idea but it's wise to ask in advance where the wine comes from and what the quality is. Don't be satisfied with being told that it is a "Merlot" or even "locally grown."
Wine by the Glass vs. Wine by the Bottle
It's not always clear whether you should order wine by the glass or by the bottle. Wine by the glass makes a lot of sense if you are the only one drinking wine or if it provides you with the opportunity to try a couple of different wines you wouldn't normally get to drink. It can also ensure that you get the right sort of wine with different parts of the meal, instead of having to make due with just a single bottle all the way through.
There are a couple of significant disadvantages when ordering wine by the glass, though. For one thing, you may not be able to get anything but the house wine by the glass and that's usually the last thing you should be choosing. Second, it's difficult to ensure that wine retains its flavor once it's been opened, so if you have to wonder how long the wine has already been opened when you order a glass of it. Is yours the first, fresh glass, or the last glass from a bottle first opened three night ago?
Finally, the cost-per-ounce of decent wine is much higher if you order by the glass than if you order an entire bottle. If two or three of you are drinking, are drinking enough, and can agree on a wine, you may be able to purchase an entire bottle for the same price as you would have paid for less wine by the glass. Given how expensive restaurant wine already is, that's a value you should take advantage of.
Wine on Ice
It's common for restaurants to put your wine bottle in an ice bucket next to the table, especially if you are drinking white wine. If the wine was merely stored in a cool place rather than chilled to the right temperature, this makes sense. If, however, the wine was already cold — and often this means being too cold — putting it in ice will just make the situation worse.
If you think your white wine is already a bit on the cold side, don't hesitate to remove the bottle; you can always return it later if necessary. On the other hand, your red wine might be served a bit too warm and in that case don't be afraid to ask for an ice bucket to chill it for a few minutes. It's your wine and you're already paying a premium for it, so you might as well be able to drink your wine at something close to the optimal temperature.
Restaurant Wine Glasses
Not all restaurants use the correct glasses when serving wine. If they don't, it's a good sign that the people in charge probably don't understand their own wines very well, so it's advisable to be cautious. Don't be afraid to ask if different glasses are available or even to substitute others glasses already at the table. It's not unusual for a restaurant's water glasses to make more suitable red wine glasses than what your red wine was served in. On the other hand, if you drinking the house wine, maybe it's better to stick with a smaller glass.
You often can't bring your own wine to a restaurant that sells wine, but a restaurant that has yet to get an alcohol license, doesn't want one, or simply can't get one (because of where they are located, for example) may have a policy of allowing customers to bring their own wine to dinner. If they do they will probably say so somewhere, but don't be afraid to ask in advance.
Bringing your own bottle of wine is usually a very good idea: the wine will be cheaper, you'll know what you're getting, it will probably be at the right temperature, you can avoid all the hassle of dealing with a wine list, and you can take the leftovers home. You'll almost certainly have to pay a small service or "corking" fee to have the wine served to you by the restaurant, but even so it's still a better value than buying the same wine from the restaurant.