To do anything easily and effectively, you need to have formed a clear mental picture, or representation, of what it is you're doing. This is especially important when writing an essay, because the writing process is often hard to pin down. Ultimately, forming this mental picture is a matter of reflective practice, in which you analyze your own thinking patterns and the ways in which you represent to yourself what needs to be done. As a writer, you need to find ways of conveying this mental picture to your reader.
Seeing is one of the clearest human senses, so it's not surprising that most of the effective essay planning techniques involve visualization. One of the best known of these is mindmapping (sometimes known as "concept mapping" or a "spider diagram"), which represents relationships between concepts in a spatial form.
Mindmapping can be very valuable if you have a visual learning style, but gives no indication of the best order in which to approach a topic, so is probably best used in conjunction with outlining, which aims to set out hierarchical relationships in which less important points are grouped under key issues, and is also probably more suited to people with a verbal learning style.
If you are finding it hard to identify distinct ideas or concepts, you may find the traditional idea-generating method of topics helpful.