[kule]

I used to have a lot of trouble with this. It didn't help having the terminology 'visualisation' which makes it feel like you're not doing it right if you don't 'see' something.

It's something you can work on by using your other senses for instance rather than trying to see something, concentrate on what it feels like, or the sounds or the smells. e.g. To picture a beach feel the sand in your toes, the breeze on your face, the noise of the waves or the smell of the salt air.

If you can concentrate with that in your mind instead I find a picture will then quite often form. Especially true if you take the time to quieten your mind first (e.g yoga, breathing techniques, meditation etc).