Hold ALT to select layered controls

Sometimes you might need to select only a few controls from your wireframe, but there's a control behind them as a background. If you just click and drag, you'll end up moving the background control instead of starting a lasso selection.

One trick to get around this is to start dragging the selection rectangle from an empty area of the canvas. There's also another way: hold the ALT key when you start dragging, and Balsamiq will ignore items behind the mouse, as shown below.