Oh man, I am a veteran at this.
The presentations are SUPPOSED to be 90 minutes. So rule #1 is to make an appointment for something else 115 minutes after the start time. Or pretend you have another appointment. Make sure you let the salesperson know you have another appointment that you MUST attend.
Rule #2 - No matter what they offer, you can get a better deal elsewhere. On Ebay, your cousin who seriously wants to dump their timeshare commitment, etc.
Rule #3 - You can almost ALWAYS get a better vacation deal than what a timeshare will cost (especially when you add on maintenance fees.)
Rule #4 - Make friends with folks who own timeshares. Make many friends. Even though I can't justify the price, some of the best resorts/condos I've been to were timeshares.
Rule #5 - If you agree to the deal, try to negotiate for more than they offer you up front. 2 days for $99? Not worth the airfare. I need 5 days min. 2 free tickets to whatever? There will be 3 of us. Etc.
Rule #6 - (Shoulda been Rule 1 or 2) Make sure the accommodations for your trip are at the actual timeshare place. Don't let them put you up in DaysInn while they make you tour some deluxe luxury timeshare. Ask them where you are staying when you book and if they don't book you on their property, tell them NO. You can get a room on Priceline or Skyauction. They are pushy, so you be pushy and demand better accommodations.
Rule #7 - This is relationship marketing. They will try to be your "friend" up front and ask you about your kids, your hobbies, etc. Tell them you don't like discussing personal info with strangers. Repeatedly ask them to "cut to the chase" or "tell me what this will cost". Keep doing this. If they cop an attitude, tell them you don't feel comfortable with them and would like to speak to their manager.
Rule #8 - If you just happen to be rolling in the dough and would really like to buy the timeshare, DO NOT ACCEPT THEIR FIRST OFFER! Decide for yourself (in advance) what you would consider a reasonable offer then make that your bottom line. You lose nothing by walking away from their offer if it does not meet what you want. A timeshare is a WANT not a NEED.