The problem with 'smudging' with the clone tool is caused by cloning from an area just set down by the clone tool. This causes a repeating pattern. It can be very obvious, as in a noticable feature or flaw repeating, or more subtle, as in the grain looking 'unnatural' since it doesn't match and is made more obvious by repetition.
The most basic answer is:
if the area is big enough where clone/stamp tool repetition might be a problem, don't use it. Use the lasso tool to circle the area with a slight feather then move to a similar area, copy, paste to a new layer, and move it into place.
But, some times that's just impractical due to the amount of work involved.
In that case there are several steps that can be taken:
- turn down the opacity and make several 'trips'
- don't drag the pointer, use several discrete clicks
- turn off the 'aligned' feature, pick an area far away that still has what you need and use separate clicks instead of dragging
- use blend modes (careful, this can make it worse)
- with 'aligned' on, keep one finger on the alt-key and pick a new source for every single click (again, don't drag)
- use the correct brush size, the smallest that will cover, and with soft edges. Use the  keys to scale up and down, and make sure you have 'actual brush size' selected in preferences
- vary techniques, vary direction, vary everything so patterns can't form
But mostly, avoid using it if you possibly can. Nothing announces an amateur restoration faster than clone tracks.