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I've seen many different ways to represent an object on a page as being draggable, but I was the most effective way in your experience of doing so?
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There are many good ways of doing this, but the best ones provide an affordance like a handle, sometimes indicated by a grid of dots or rule lines, along with the cursor change to a hand. See the example from GMail, but note that the way it works in GMail is not draggable; it is used to indicate "Move This". I'm including the image because I like the affordance, despite the fact that the implementation is not straight-up drag-and-drop.
The second example is from Edx.org, for a course called Energy 101. The instructions say "drag each tile to rank industries..." and the handle is composed of horizontal rule lines. The cursor also changes to a hand on mouseover.