If you aren’t familiar with the Manual settings on your DSLR camera, use the Aperture Priority for the best results. Some cameras have a Scene Mode for food that makes the food look fresh and vibrant.
Aperture Priority Mode will allow you to select the appropriate aperture (lens opening) and your camera will do the rest. In low light situations, it is best to use a wide lens aperture (lower number f-stop) on your camera. This allows more light to enter the camera and reach its imaging sensor, and allows using faster shutter speeds as well. If your lens can open to apertures like f/2.8 or f/3.5, this will allow much more light than an aperture like f/8 or f/11, for example. If you're using a standard zoom lens, you may not have apertures this wide, but use the lowest f-numbers your lens will allow. With these wide lens openings, you sacrifice some depth-of-field, and you will notice an increase in blurriness in your background. This blurriness can actually be an advantage in cocktail photography, forcing the viewer's eye on the cocktail, rather than the background information.
Ideally, a tripod will provide the steadiest platform for your camera, and give you the greatest chance at sharp pictures. With a tripod, it's a good idea to use your camera's 2-second Self Timer option, so the action of pushing the shutter button doesn't cause camera shake or vibrations.
If you don't have a tripod, resting your elbows or the camera itself on a solid, flat surface can help with minimizing the risk of camera shake. Lenses with Image Stabilization give you yet another advantage in hand-held, low-light shooting, so be sure it's turned on if you have this feature.
Low light shooting normally means using high ISO settings, but remember that the higher the ISO is, the more digital noise you'll end up with in your pictures. With a tripod, it's easier to stick to lower ISOs (like 100 or 200), and the resulting slow shutter speeds, and still get good, sharp pictures. Hand-held, you'll probably need to raise ISOs in low light, but don't immediately jump to your highest ISO settings. Try settings like ISO 800 or 1600, and see if you get reasonable shutter speeds at those wide lens apertures.
Use natural light to really capture the ambiance of a cocktail bar even if you are photographing at home. If you find that the scene is too dimly lit for your camera, then add votive candles to enhance the existing light. To really make the cocktail glow you can place a single votive candle directly behind the glass if it doesn’t have a stem. This illumination will turn any colored drink into something mesmerizing to the eye. With clear drinks, place the candle to the side, so light bounces off the liquid. You will have a highlight side and a shadow side to your scene, which is far more dynamic than a cocktail that is evenly lit. With colored, milky or bubbly drinks, you can place the candle behind the glass to exaggerate the appearance of texture and bubbles.
Use a bounce card (white card or paper) from below and in front to add light to the front if necessary. This will add a subtle fill-light to the image and is very helpful in situations with incredibly low light.
How close should you be to the cocktail? If you have a good macro lens on your camera, then you can get as close as you want. If you don’t have macro capability, or are using a point-and-shoot or smartphone, then the closer you are to the cocktail, the less control you will have on the focus. Put some space between your camera and your drink for best non-macro lens results. A distance of two to four feet away from your subject should be plenty of space to allow your lens to properly focus on the cocktail and help you to achieve the best results.
Try photographing your cocktail from a variety of different angles. Remember that although your cocktail is a stationary item, you can move 90 or even 180 degrees around the cocktail to find the best angle and light.