For stuff I like that I just edge out backwards until I can see a sliver of the side of the block I want to stick another to.
..........O right here I can just barely see the right edge of the block, so I can place another.
[ ][ ][ ]<--place new block here.
(side veiw)
It takes some getting used to, but can be used fairly reliably.
It's a lot easier if you do something at least three blocks wide, though, then you can look sideways from one side to the other and place a block on the far side. Four blocks would be even safer, and then you could slim them down to one if you liked
............[ ]<--new block
[O][ ][ ][ ]
(top down view)