Pick a date
-- Pick many dates that may work well for the event. Try not to conflict with other main events on your city, especially another dance event.
Research Locations
-- Most venues tend to book any where from 3-8 months in advance. Good venues (for dance functions) are slim picking in some cities. Your are looking for a decent-sized dance floor with minimal posts in the main dancing area. Square footage is important depending on how may people attending the function. And one of the most important things is airconditioning / large fans or lots of windows for air circulation. Especially in the summer!
Find an instuctor
-- Do your research. Find out what they charge and if they can be booked for the weekend of your choice. Some instuctors may be booked a year in advance. Check their availibility.
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Some instuctors are exceptional Ballroom instructors, but if it's Argentine Tango that you want, then make sure you research instructors that teach the dances you want taught at your workshop. (Remember- you don't want your Waltz to look like a Salsa or your Cha Cha to look like a West Coast Swing.)
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Another main consideration is the Level of instuction! If the people you are signing up for the workshop are all beginner or novise, then to bring in a world champion is overkill! (Unless they live next door.) You don't have to fly someone in from England if you have access to qualified instuctors within a reasonable distance.
Important Note:
-- Picking a date, researching the venues as well as the availability of the instuctors all go hand in hand. You may want to have three or four dates to work with when you book the hall and the instructors as some of the dates you want may not be available to you.