During meditation you are often required to visualize something, be it the object of your desire, lights or enlightened energies etc.

Basically the more clearly you can see things in your mind's eye, the better. However it's a fact that most of us can't visualize very well. I believe we can get better with practice but in the meantime we need some "workaround" to keep us motivated and help us improve our results. The best advice I was given is this: even if you can't see the object of your visualization clearly you have to know it's there.

For example, look at something in the room, say the desk in front you. Even if you looked away you would still know it's there, right? Of course, there will be no doubt about it. If you can apply the same kind of attitude towards your visualization, your practice will be successful.

Finally “visualization” is not limited to mental images. If for example you have the ability to hear sounds clearly in your mind or to experience sensations, then do that. Do whatever you can to make you picture as vivid and as real as possible.