There's nothing worse than shooting for 10 hours and later discovering that the last 8 hours of shots are unuseable because dew has formed on the lens element. There are a number of ways to prevent/reduce dew formation but first we'll look at how to determine if dew formation will be a factor given the current weather conditions. 

We need to find an app or website that shows the current conditions, including the dewpoint. I use an iOS app called AusWeatherHD.

As you can see the current temp is 16.3deg celcius. The dewpoint is 5.9deg and humidity is low at 50%. Wind blowing across the lens will also hold off the formation of dew, there's not much wind around tonight but that's not a problem given the conditions, there's pretty much zero chance of dew forming on the lens tonight.

When the current temp drops below the dewpoint thats when it becomes a problem. The solution to this is to keep the lens temperature slightly above the temperature of the air around it. There are a few different methods for this.

Powered Heaters

The best method is a powered dew heater. I use a Firefly strap from KendrickAstro, connected to a lithium-ion 12v battery. They also have a line of Premier heater straps, these run hotter but also use more power.

The dew straps are often used with a controller unit, but I've found that there isn't any advantage to using one. The controller can adjust the power going to the strap, I found I was always running mine on full power anyway. The Kendrick controller I used had a low voltage cutoff, so when the battery output dropped slightly it would kill the power to the heater strap.

I was getting a run time of around 2 hours, requiring 4 battery swaps during a 10 hour shoot.

When connecting the strap directly to the battery without the controller I'm getting about 5 hours run time, that's only one battery swap in 10 hours.

If I'm shooting somewhere that's not too far away from my car I'll use this as a power source:

It's big and heavy but it will run the heater all night, not good on long hikes though.

Chemical Warmers

Before I got the powered heater I used this method. It's better than nothing but requires constant attention to be effective.

The ones I normally used were called hand warmers, the supplier ran out of them so I got toe warmers instead, which are slightly larger and have an adhesive pad on them.

They are a small sachet containing activated carbon, when they are removed from the packet and exposed to air they produce heat. When used for their intended purpose in a shoe or glove they generate heat for 5 or 6 hours, but when placed on a lens barrel where they are not moving around they will only output heat for around an hour.

Ideally you would join 3 of them together so the top and sides of the lens are covered, you need to be very careful when changing them not to move the camera or the zoom & focus rings.

Fans

I tried this method briefly, using 100mm PC fans powered by a USB battery pack to move air across the lens element. I shoot at 14mm, which made it hard to position the fans close enough to the lens to be effective without them being visible in the frame. For longer focal lengths it could be a good option.