Maintaining Altitude

[MeditationEye]

I had a really vivid flying dream a couple of nights ago, flying through a valley over the greenest fields and trees. I tried to climb but kept sinking. It worked once or twice but couldn't get higher than the hills on either side of the valley.

That always seems to be a difficult task for me - maintaining height. I tend to gentle sink back down.

[StillMoving]

You gave climbing a definition, and sinking as its natural opposite.
As you aim to climb, you fear sinking. Which could be distracting.

[Tobi]

Unless you are sure this was an astral projection/out of body experience, then a flying dream doesn't necessarily mean you are conscious or functioning on an Astral plane.

Usually it is an experience of partial awareness of being outside of the physical. Very often when waking up suddenly from a flying dream, one will be in the vibrational state, maybe only partially separated from the body, or just about to be.
This is translated into a dream in a greatly exaggerated way very often, when even a slight partial separation can result in a vivid dream of flying over landscapes etc.

That you keep sinking during flight in the dream, could be that you are not yet able to maintain consciousness outside of the physical state, without a magnetic pull back to 'consciousness in the body' (or back to your physical waking.) That magnetic pull back to physical body consciousness is very strong.
If you practice out of body techniques, or become more used to going out of body, you may become stronger at being able to stay out for a longer time.
You have to really really want to, as it takes a lot of energy, also a certain lightness of being....and the other thing you will need is to feel very very secure subconsciously, about it.