Backlight, where the source is behind the subject can create quite a sparkly image, however, it also causes problems.

When in a heavy backlit situation, try rotating your subject 3/4 into the light.  And have an assistant bounce some light back on them with a white card.

If it's a backlit building for example, you're best to come back later.

These are the little tricks that us pro's deal with on a daily basis to get great images!