Book your trip to Ibiza during the tourist season. The party season runs from the beginning of June to the end of September with only a couple clubs open into the first week of October. During the summer months, promoters host nightly events at all the big clubs with pre-parties at the smaller bars. In the off-season, mid-October to late May, the only establishments open are smaller bars and one large club (Pacha).
Narrow down where you want to party on the island. There are two major resort areas that feature the famous Ibiza nightlife- Ibiza Town and San Antonio. Ibiza Town, near the harbor, is popular with Italian tourists and is home to several gay clubs. San Antonio, or the West End, is popular with English tourists.
You can find good overviews of the major clubs and several of the smaller bars on several websites. The official travel directory of Ibiza also has a good directory of the larger clubs. These websites may also provide club reviews, party calendars & 'clubbing forums' that provide up to the minute information on what's hot. Each club has its own personality and claim to fame. Es Paradis is known for its 'Water Party' while Amnesia features its 'Foam Party on Tuesday nights. Search until you find a place that suits your party needs- be it dancing, drinking or just looking at beautiful people.
Once you're on the island, your best nightlife resource is the advice of others. Definitely socialize and ask where other people have been. The major resort areas are full of gorgeous people distributing flyers promoting parties. Many of the flyers offer free or reduced admission to clubs, free drinks or other tips on pre-parties. Take the flyers and check them out, this could lead to the best party of the night.
Have Fun!!!