Know the situations in which it is appropriate to bargain. Not all situations call for bargaining. A bazaar in Morocco may be a great place to bargain, but Harrod's in London probably isn't. What's acceptable is one place is bad shopping etiquette in another.
If you want to know whether it's acceptable to bargain, say something casual like "It's just a little too expensive for me." If the merchant makes a counteroffer, he's effectively opening the door to bargaining, in which case, haggle away. If he stands pat, it's probably not okay to bargain at that location.