When working on dense designs, or on designs where you have items that 'overlap' each other on different layers, it can be difficult to select the right item.
To help with this, you can use the 'Select Next' option (available on the right-mouse 'context' menu) to spin through all the items at the cursor position.
For example, if you have two track segments, one on the top and the other on the bottom, that run along the same path, select the first segment by simply clicking. The segment becomes selected (highlighted). Now right-click and choose 'Select Next' (or the appropriate shortcut key) The program ignores the item already selected, and instead selects the second track segment, the one on the other side of the board.