Depending on where you bought your Disney tickets, register for My Disney Experience and link your tickets. The free park photos are great (be sure to download them as soon as you get home, they're not permanent) . Book your fast pass for Frozen, 7 dwarves, Avatar now if you can (they may already be full up). The queues for the really big rides will not go down until the park closes (although this will depend on how late the opening times are) so if you can do a big ride early when the queue is <40 mins, I'd do it. The disney app is also great and all the parks have free wifi, watch your battery levels (may be worth investing in a 20,000mah battery pack for people to do emergency charging).
I'd personally try to pick the least busy day for magic kingdom (they're all busy mind) but I understand wanting to do it first day. Remember to allow 25+ minutes at Magic Kingdom to take the boat in if you're aiming to get there early.
You will do incredible amounts of walking at Epcot without realizing it, make sure the day after is easy. Despite what I said earlier, Spaceship Earth does start off the day packed and get quiet in the evening (plus it's a great way to sit down for 15 minutes and relax). Soaring is a ride that also builds up massive queues that stay high all day. Disney tend to underestimate their queue times, be careful not to miss the fireworks.
At Universal be aware that Kong breaks down a lot. Don't leave it too late in the day. The train between the park is a ride in itself and is pretty clever (different experience each way). You get extremely wet on Popeye and Dudley Do Rights, far wetter than you tend to get on British rides so those are obviously best done at the hottest times. The novelty drinks are very expensive but worth trying; Flaming moes are great for Simpsons fans and raise a smile, Butterbeer is very nice (there are usually people serving from both sides of the butterbeer cart, often people queue at one side and the other side gets ignored).
On your first evening, pop into Walmart, get some snacks, drinks and a carriable water bottle that you can use in the parks. Golden Corral breakfasts will mean you don't feel the need to have a full lunch at the park but your hotel may do its own buffet. Remember to budget $20 a day on parking. Lots of US petrol pumps require you to flip a lever to turn on the pumps rather than it being automatic or done by the attendant (catches a lot of people out). The I4 when it's busy in the evening is very hairy (traffic passing on both sides, speeding trucks and terrible American drivers in general).