Most commercial photographers who use smoke a lot buy a commercially made "fog machine" that converts a fluid into dense vapor. The fluid is primarily made of water and glycerin. The machine simply heats the fluid in a semi enclosed space, producing the fog/smoke.

You can create such a machine yourself easily. You just need something that can hold the fluid above a heat source. An enclosed vessel with a hole in the top does better than a vessel with an fully open top, as it allows the vapor to collect above the fluid before escaping.

This DIY YouTube video shows how to cut apart an aluminum soft drink can to make a simple fog machine heated by a candle placed underneath it. Basically, the can is cut in half. The lower part is inverted with the concave "dimple" on the bottom holding the fluid and most of the side is cut away to allow a small candle to have a source of fresh air. The top half of the can is then placed on top of the inverted bottom half with the small hole in the top open to allow the fog to escape.

This DIY instruction uses a mini pie tin with the inverted top cut off a two-liter soda bottle taped to the top of the pie tin, which is suspended above a large candle. This YouTube video makes a similar device.

The trick with any of these DIY devices is to only use a small bit of fluid, say a teaspoon to a tablespoon (5-15 ml), at a time. You'll be surprised at just how much "fog" will be made using such a small amount of glycerin and water!

Instead of buying commercially made "fog juice", you can make your own with distilled water and glycerin. Your water must be pure and free of chemicals such as those often added to tap water, like chlorine, fluoride, and various softening agents. Vegetable glycerin will work if no chemical grade glycerin is available. I've seen ratios of anywhere from three parts water to one part glycerin (25% glycerin) to a glycerin:water ratio of 7:3 (70% glycerin). Most sources go with about 25-35% glycerin content.