Don't look at it.

Really--sight is honestly the big issue here, you can't hear or taste or smell the wall, and it's the fact that you see it that you expect to touch it. I've gone backwards through walls and floated through ceilings while looking down plenty of times before. It's fairly trippy, but it works.

If you, however, want to overcome your sight, then think of it this way: There really is no wall there. Your subconscious just feels like putting a simulation of a wall there, but it's your line of sight, and in a dream, your line of sight is taken from nothing: it's just, kind of, placed there. In reality there's a thing there and your brain interprets it and gives you the information through your five senses, but in a dream this data is pulled from nothing, it's just kind of... made up. This means that dreams really have nothing in them, so by theory you should be able to just... do whatever you want in that nothing. That wall is just your imagination.