If you use the rectangular or oval marquee tool to make a selection, you can use the spacebar to reposition the starting point while dragging! Lemme 'splain:
Step 1. Select the marquee tool.
Step 2. Click at a starting point and drag the cursor in a diagonal direction, thus creating a marquee selection stretching from the starting point to wherever the cursor is.
Step 3. Hold down the spacebar. The selection will stop changing shape and will move around with the cursor.
Step 4. Release the spacebar. The selection will go back to changing shape, stretching from the new starting point to the cursor. Repeat as necessary.