Traditional Starch ThickenersRegular Flour - White flour is the most common thickener used in sauces. In some recipes, you might be fine using it. There are 6 grams of carbohydrate per tablespoon of regular flour. This will thicken one cup of a gravy (which has some thickeners from the meat), or a thin sauce. It takes two tablespoons of flour to thicken a sauce of medium thickness, and three for a thick sauce. Whole wheat flour has 4.5 grams effective carbohydrate plus 1 gram fiber per tablespoon. It takes slightly more to thicken a sauce than white flour.
When you use flour to thicken a sauce, you can't add it directly, as it will create lumps. The best way to add it is in a roux, where you heat it with a fat like oil or butter and cook it for a minute or two (stirring constantly) to get rid of the raw flour taste. Then whisk in the liquid. A roux will slowly get darker the longer you cook it, and some recipes will call for darker rouxs, however, their thickening power decreases as you cook them, so for low carb purposes, a white roux is best.
Other ways to add flour to a sauce are a slurry, where you stir or shake up the flour with cold water and then add to the sauce, or a beurre manié, where you mix the flour and butter into a paste and add it bit by bit. You can also add the flour to vegetables or meat as you are browning or sauteing them.
Cornstarch - Cornstarch has 7 grams of carb per tablespoon, but more thickening power. According to the corn starch manufacturers you only need half as much cornstarch as flour, but experts seem to vary on this point. Sauces thickened with cornstarch are less opaque and glossier. Cornstarch is generally added to cold water and then to the sauce. You don't have to worry about cooking it first.
Arrowroot - Arrowroot is similar to cornstarch and used the same way, except it makes a totally clear sauce and lends a glossier appearance. Stands up to acidic liquids better than cornstarch.
Making a Low-Carb White SauceTo make a low-carb white sauce (Bechamel sauce), you can use any dairy product (see carb counts), or unsweetened soy or almond milk (check the ingredients carefully to make sure no sweetener is added), with any of the thickeners on this page.
Alternative ThickenersIf you want to avoid starches altogether, there are quite a few alternatives. Which one you choose will depend a lot upon what you're making.
Reduction - Simply simmering the sauce until enough water evaporates that the sauce is thicker works in some cases.
Vegetable gums - Yum! They may not sound really appetizing, but vegetable gums are just a type of fiber that absorbs water to make a sort of gel. They are often used as thickeners in commercial products. The most common are guar gum and xanthan gum, which can be purchased at most health food stores or online.
To use vegetable gums to thicken sauces, sprinkle them into the sauce while whisking. Often it only takes a small amount, so go slowly. Too much will overthicken and/or give the sauce a "slick" feel.
An alternative to plain vegetable gums are products based on them which are a little easier to use. Expert Foods makes several of these products for different uses. I particularly like the not/Starch.
Pureed vegetables - This is especially good for creamy soups, but works for other sauces as well. Almost any cooked vegetable can be blended and used to thicken a soup or sauce (think broccoli or pumpkin soup). Tomato paste is a great thickener. Eggplant, zucchini or other squash, cauliflower, or the lower carb root vegetables are all excellent choices when you don't want the vegetable to add too much flavor.
Cream - Cream will thicken as it reduces, so if you add cream to a sauce and boil it, the sauce will thicken more than reducing without the cream.
Sour cream is already thickened - whisk it into a sauce.
Cream cheese is thicker than sour cream, although it obviously has a distinctive flavor.
Butter - If you add cold butter at the end of cooking a pan sauce, it will have a thickening effect
Egg Yolk - Think of mayonnaise - at it's heart, it's nothing more than oil and egg yolk. Or think of hollandaise sauce. An egg yolk can really bring the right kind of sauce together, especially if there is oil or fat in it. Don't add the yolk directly to a hot sauce or it will scramble. To avoid this, "temper" the yolk(s) by adding a small amount of the sauce to the them to gradually bring them up to temperature. Then add the tempered eggs to the sauce.
Nuts and nut butters - Ground nuts were traditionally used to thicken sauces in olden times. Nut butters (peanut, almond, etc) work even better.
Flax seed meal - Flax seed meal does thicken liquids, but it's so grainy that I haven't found it to work well in many sauces. I like it to thicken shakes, though.