Example of a positional sacrifice

Black to move:

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My opponent has just played 19. f5.  This looks scary because if my g pawn ever captures on f5, then Qh5 and mate to follow.

Note that Black is already up a piece [due to Knight sac on g5] so I decided to return the favor with 19. ... Bxg5!! and, after 20. fxe6 fxe6 [20. ... Nf6 is optimal], it is now Black's turn to attack, with the open f file and the White King unable to long castle due to the Bishop on g5.

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For me, tactical sacs are much easier to see:  those that smash the enemy's defense and allow a potential checkmate attack.

Positional sacs are much more subtle, like "quiet" moves: you're giving up material for some positional consideration, which might be difficult to judge [in this particular case, it wasn't that difficult.  The most difficult part was accepting the fact that I could not hold on to the extra material and that returning it was the best idea.].