It’s a good idea to count cards at spades. (See Pagat.com for Spades rules.)
The trump suit, obviously, is always spades. You can win by either having a higher card of the same suit that has been lead or by having a spade.
Therefore it’s important to have an idea of who’s got certain kinds of cards and especially what kind of cards have already been played.
The least you should do when counting cards at Spades is tracking the highest card values of each suit — if you’ve seen the king and queen of hearts, you know the highest heart is a jack at the moment. This will help you a lot especially if you hold the jack of hearts.
It would also be useful to know if your opponents still have certain suits left or not. Knowing which player is out of a certain suit, for instance, can help you greatly. You can use all kinds of information to your advantage in order to make educated decisions. Good guides to playing Spades are Master Spades and Howtodothings.com.