The parks will be very crowded first of all so my #1 tip is crucial.
Here are my top #10 tips for WDW vets
#1 tip. Get there EARLY! There are NO long lines in the first hour-hour and a half. Pick up a FP and enjoy some rides without lines
Use FastPass! (same kind of system as WDW)
Do not expect WDW size. DLR is so compact it is very easy to get around. Park hopping (or going to Downtown Disney) takes a grand total of 1 minute to cross the large plaza (park hop) separating the two parks. It also makes walking the main transportation here, as opposed to buses.
This tip does not matter to you since you've already decided on a hotel. I do not recommend staying onsite for a combination of two reasons. Cost and closeness. There are many many many offsite hotels with much cheaper prices. Also there are a handful of hotels that are closer to the parks than the
Disneyland or Paradise Pier Hotels. With cheaper prices and shorter/equal walk, I have always stayed offsite. While the quality may not be as good, it's just a place to sleep right? However if you intend to spend a lot of time at the pool or lobby or hanging around the hotel, you might be the kind of person who will want to shell out the dough, though I highly discourage it. More money for the parks!
Buy admission in advance definitely. Nothing like standing in another long line just to get into the park.
Eat breakfast at your hotel, or while waiting for the gates to open. Plus, breakfast in the park is mucho $.
Dont miss spectacular shows such as the Aladdin Musical at DCA and the nighttime shows Electrical Parade at DCA (both unique to DLR) and the nightly fireworks show and Fantasmic! at DL.
Do not eat lunch and dinner at normal hours to avoid food lines and take advantage of shorter ride lines. During busy holiday season, some food lines can get to be 20+ minutes long.

I highly recommend eating at the Blue Bayou. It's a sit down restaurant overlooking the Pirates ride. Though it does require reservations (particularly in busy season) and it's not cheap, it's very worth it. Some good counter service places include: Hungry Bear Restaurant in Critter Country, El Rancho del Zocalo in Frontierland, and Redd Rocket's Pizza Port in Tomorrowland. Also check out the dining options in DTD, just a minute's walk away.
An alternative to carrying around all your stuff all day is to rent a locker. Theyre about $5 each and can hold a little more than a backpack size. Perfect for storing sweatshirts during the middle of the day. Lockers are located about half way down Main Street to the right in a little corner at DL. At DCA they are just to the right of the entrance. THis also helps a lot during the winter months when it gets a bit chilly in the mornings and evenings.
Have fun!