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Hi Orestis,
Dew. My old friend, and arch enemy.
John's already mentioned the dew strap solution. I've got a few straps rigged up to my big dob. The thing is that the more straps, and the longer they are, they too suck up the big ones from the battery. Yet there is no real way to deal with dewing eyepieces, secondaries or finders unless its with straps. A hair drier is only a short term solution, though a good one to start with. Camping supply stores is where you can find the 12V ones, but these are very hungry beasties too.
240V driers are very fast, but be very careful with them in a dewy enviroment. You need to keep them off the ground & under a box when not in use, and the plug protected so water doesn't get into it.
I've been seeing a few dobs today use small 12V fans to blow air around the primary. SDM Telescopes has been installing these onto their dobs. The bonus is they use much less juice than straps on a big primary. Maybe ask them for a few tips on these.
Another thing that has helped me keep my eyepieces warm when not in use during the night is keeping a hot water bottle in the eyepiece case. You may need to change the water once or twice over the course of the whole night, but a thermos filled with hot water will see you right. The hot water bottle doesn't need much water in it either. Also if you do use straps down the track, the EP heating strap will be more efficient if the EP is already warm to begin with.
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