When you set your Explorer to Details view, it can become tricky to drop an item onto the window background (in order to move it into the folder) because Details view sets full row select (starting in Windows Vista). This helps users of tablets and touch screens, because it increases the size of the target when dragging and dropping into a folder. On the other hand, when you have more items than fit on the screen, every pixel in the view corresponds to an item; there is no background any more. So how do you drop on the background?
If you're using Windows 7, you can take advantage of a little gutter space to the left and right of the columns. There is a gutter to the left of the leftmost column and another gutter to the right of the rightmost column. Windows Vista doesn't have these gutters, but you can create a gutter on the right hand side by resizing your columns narrow, and then resizing the window wider than the sum of all the columns. Yes, it's clumsy.
Less clumsy is dropping onto the Details panel at the bottom of the window, which works both on Windows Vista and Window 7. Dropping onto the Details panel is treated as a drop on the folder background. And the Details panel is a nice big target for your drag/drop operation, so those of you using a tablet or touch screen should have no trouble finding it. (Those gutters are nice for mouse-based users, but if you've got fat fingers, they're not much help on a touch screen.)
If you've turned off the Details panel, you can also drop onto the title bar. It's not as big as the Details pane, but it's still bigger than the gutters.
Finally, there's always the copy/paste approach instead of drag/drop. Multi-select the items you want to transfer, right-click them and select Copy (or Cut, as appropriate), then go to the destination folder, and under the Organize menu, select Paste.
Except for the gutters, all of these techniques also work for Windows Vista.
Bonus chatter: Commenter Medinoc complains that, with the switch to full row select, it's very hard to click on the background. Actually, it's not that bad. You can right-click on the blank space of any unselected item. For example, find an unselected item with a name that doesn't fill the entire Name column and look at all that white space to the right. You can right-click there and it will be treated as as a right-click on the background. The full row is still treated as part of the item when dropping, for the benefit of people using tablets and touch screens, or people who simply want to hit a target bigger than a 16×16 icon.