Avoiding the gap...

I often see people complain about the gap which can exist in their crochet squares.  Here's the solution I found:

First of all, after the first round, rather than do the standard ch3, only do ch2. 

Rather than go into the top of the ch_ of the previous row, 

you go into the front two loops of the stitch to the left of it. 

Then ch2 and go back into the same space.

Once your square is finished, this is the way it will look.  The arrows indicate that your join ch and first stitch of each row are in the top of the same stitch from the row before.

Et voila; no gap.