This is a link to a site called Undercover Tourist.  http://www.undercovertourist.com/united-states/florida/orlando/attractions/when-to-visit.html  They have a chart of the busiest months and busiest days at the parks.  Having been there at most times of the year myself, I'd put my 2 cents worth in to say I agree with their assessment. 

First off, one of the ways you can get discounts on most things Disney and free parking (which now amounts to $12 per day) is to buy a yearly pass.  I know this is a big investment, especially for families but if you plan on going to Disney more than once a year, you will save enough to make it worth while.  The biggest mistake you can make is thinking you can do all this in one trip!  You need down time to chill in the pool or take a boat ride from Down Town Disney (more about this later) or everyone ends up cranky!  The greatest thing about the passes is that you can go to Epcot just for a couple rides and dinner if that is all you want that day.  No pressure to stay the whole day because you paid big bucks for that one day!  You can park hop or arrive early, leave for a swim and lunch then go back for fireworks and not feel like the clock is ticking on getting your money's worth from your ticket for that day.  This is not for everyone but for any Disney philes like us, that want to go back again and again, it's something to consider.  Pass holders also get discounts on the Disney Dining Experience card http://www.allearsnet.com/din/dde.htm which saves you 20% on almost all Disney dining. 

The Undercover Tourist site also has discounts on attraction tickets http://www.undercovertourist.com/united-states/florida/orlando/attractions/tickets/.  I have never used this company so you should do your own "due diligence" as to whether they are on the up and up.  I have been told that the Kissimmee Walmart also has discounted Disney tickets so you might want to call and see what their prices are like.  This is another ticket site http://www.ticketsatwork.com/affiliates/nextjump/pages.php?sub=wdw again, I have not used them and I question their "corporate discount" tickets.  These are supposed to be for large sponsors of Disney rides like Siemens and Kodak.  If you work for one of Disney's large sponsors though, definately check with your employer as they might have significant discounts available for you.

Rental Cars

We've had good luck as U.S. renters with Alamo.  They are located at the airport, just a short walk across the street from the terminal.  Check out www.alamo.com and click on their specials.  They usually have a coupon for a weekly rental which you can get the code for by clicking on that link and noting the coupon code.  Then go back and do a prepay (there is a charge if you cancel when you prepay so be sure to check that out as well) which saves you about 10%.  Add the coupon code and they will take off the additional $20 or whatever the discount coupon is at the end.  When you get the car quotes, it will say "coupon will be applied" to let you know which cars are eligible.  Alamo has kiosks at the rental counter which are rarely used.  If there is a line but no one is at the kiosks, feel free to go around them and do a quick check out at the kiosk.  It can save a lot of time!

The other 2 car rentals you might use for comparion are www.dollar.com and www.buget.com .   We tried Advantage rent a car once which gave us a great deal but you need to catch a shuttle to get your car and the shuttle never showed up!  We called and were told they were out of cars and to go to L&M or EZ and they would match the price, which they did.  I don't think I'd rent off the airport again.

I don't have any suggestions for visitors from the UK or other countries.  Maybe if you know of a company you have used before with good rates, you'd let us know and we'll post it here.