Mega-dittos to Tinaluis' post. Avoid purchasing any rental car company's extra insurance "protection." Unless you have no auto insurance or are woefully underinsured, you don't need it. It is a very good idea to take proof of insurance with you (your insurance card should be sufficient).
Some credit cards (check with your company) may also provide secondary accident insurance when you charge a rental car with the card.
As for upgrades -- do you really need the extra room? If not, don't go for it. A polite "no thanks" to the offers for insurance and upgrades will usually do the trick.
Another catch-- pre-paid gas. My opinion -- this is a bad deal. You pre-pay for a full tank and bring back the car empty. However, I've never been able to figure out how to use up all the gas so I roll up to the return area exactly on empty. They do not refund the difference, so you end up paying more than if you just fill up the car yourself.
Last week I rented from Alamo for a business trip in Orlando (argh, no Disney!). It cost $10 to refill the tank vs. the $22+ they wanted to prepay.