Sometimes it's difficult for me to remember to add strong opponents to my protagonist's goals. I just don't like 'villains' in the kinds of stories I'm writing right now. However, I know I need to remember that antagonists don't equal villains.
These characters are in conflict with the protagonist over the same goal; that has nothing to do with their moral alignment or personality. The antagonist can even be a hero in his or her own right.
I have to remember that.