Report items affecting each others positions in SSRS 2008

Another approach: create rectangles containing the fields you want to hide, then put a WHITE line across the entire bottom (or top) of the rectangle. This keeps the rectangle the same size even if the fields within it are hidden. A rectangle will shrink if the fields within it shrink, but the line spanning the entire rectangle prevents this from happening.

In the side by side example, you can place the control that may be hidden into a rectangle big enough to hold both; but then put the other control that must stay in a fixed position BEHIND the rectangle (i.e. the parent of this control is the report rather than the rectangle).

If that's not enough, and the hidden and non-hidden items use the same dataset, you could (shudder) use a giant tablix control with a grouping of =1 and the right number of rows and columns to control the layout (because you can insert rectangles inside).

Some other tricks are to set the color/backgroundcolor properties to white (though in some rendering formats you can click and drag to highlight and still read what was there)... alternately you can go so far as to make the contents of controls all expression based, so as not to produce anything if a condition is met, and so it's basically invisible.