I would start emailing the super clubs in January/Febuary time if your seeking to work for the big guns, this is what i did and ended up working for Meganite
On the other hand, if your just seeking to work @ a bar in san antonio, or other parts of ibiza, simply turn up with a few $$$ and get asking, most book a holiday for a week or 2 so if nothing turns up they have a flight to return home on, most common is PR'ing tho i wouldnt advise it, i done it a coule of years back, then wasnt so bad, but after seeing how ontop the police were last summer, its just to much hassle!
Get to ibiza late may, Barwork would be availible also...
Enjoy, its a special experience! 8)