what were your experiences with demons in the astral plane?

[yondaime109]

How do you fight them?

is there something you can do to avoid them?

[Lionheart]

How do you fight them?
is there something you can do to avoid them?

Don't go looking for them, then you won't have to fight them. Once you learn to Astral Travel you will find there are different tiers. Your "demons" would be would be found in the lower level of these tiers. As you develop further you will move on to higher tiers where the lower entities can not go.
In the Astral thought =actions, so whatever you create in your mind will be there. Go in a with a positive mental attitude and you will find positive things there.

[shineling]

To avoid negative astral projections you should take up a regiment of meditation. That helped me. I meditate on the Light within my third eye and listen for the "sound current". Not doing drugs helps too or alcohol. Clean body and healthy mind equal positive astral projections. 

[CFTraveler]

How do you fight them?

is there something you can do to avoid them?

Yes.
First, know that most of the negative entities you encounter while nonphysical are not demons, they are usually either astral wildlife (elementals) and basically harmless (but scary) or your own thoughtforms- projections of your own fears. 
Demons are so different than us that they usually don't bother with us, unless we're 'special' in some way, and I don't know many people that are.
So how you avoid them is to not focus on fear or pain, and if you do encounter something negative know that you have the power, and you can always wake yourself up if you can't avoid them or are too afraid.
Yes, there are things you can do (such as using salt, water fountains or keeping a clean clear space) to keep you feeling secure, and I practice some of them, but knowing you are safe is the best defense, period.