If you can accord it, I recommend Expression Blend if you are going to be doing much UI design. It allows a simpler view of the items. Nesting controls into various containers is a good way to get the UI to be dynamic, but structured.
Typically I will use a Grid panel to break a window up into functional areas. Then I will use a series of StackPanels (often a vertical stack panel with horizontal StackPanels inside it, each with a label and textbox).
Unfortunately Grids only work as you stated. Elements in them specify the row and/or column that they reside in. If you used Blend, adding Grid Columns or Rows will have controls auto-magically change the row/column specification to stay in the position they were placed.
Hope it helps.
Expression Blend Trial at Microsoft.
UPDATE:
VS2012 has a lot of the Expression Blend functionality baked into the WPF designer. Much of the need for a copy of Blend is no longer there as developers have access to a lot of the cool tools from Blend.
