Imagine that these happy children are stitches.

The girl stitches on the left are happy Left Slanting Decreases

The boy stitches on the right are happy Right Slanting Decreases

The boy in the middle is an unhappy Left Slanting Decrease. Poor kid.
For a decrease to be happy, it must be supported by it's friends.

Right Slating Decreases are supported when they lean against existing stitches in the same row. Left Slanting Decreases are unsupported if they don't have friends to lean against.

So the trick is to work Left Slanting Decrease in the following row, so that the decrease can lean back against stitches that have already been worked!

Here's how I do it:

Step 1: On the Right Side, knit the stitches to be decreased singly (not together), wrapping the yarn in the opposite direction (counter clockwise) to seat them correctly for the next step.

Step 2: On the Wrong Side, work to the sts to be decreased. They will be obvious because they'll be seated differently on the needle (due to the way you wrapped them in the previous row.) Thus, they will be perfectly oriented for you to go on to Step 3...

Step 3: Purl these two sts together through the back loop. Insert the right needle into the stitches from the back as foll: Insert needle into the second stitch from the point (pink st), then insert it into the stitch closest to the point (yellow st). Purl them together. (Click on image for animation)

You have just created a much happier decrease that is Left Slanting when viewed from the Right Side of the work.