Paying for the trip:
IF YOU ARE GOOD WITH CREDIT CARDS you can save some money here. I am good with credit cards, so we have several that are travel-branded and i save money here. However, you have to pay them in full every month, and they need to have no (or very low) annual fee to be worth it. Right now, I have enough Disney VISA points saved up to pay for 4 adult and 1 child, three day park tickets, for our trip in May. We are also flying for "free" as I have enough Southwest points saved up; all I had to do was pay the taxes on the flights (so <$23 per person for airfare). To get to this point, we signed up for the cards when there were bonus incentives and met the incentives, and then we charge everything we buy (that we can, without an additional fee for using a card) and pay the bill every month, in full.
I have memberships at Sam's and BJ's, so I can save on buying Disney gift cards. Both stores give you a small discount on any of the cards they stock, but I wait until they run special sales (usually in November for Sam's, but also other times) and buy gift cards then. I find it saves 5%-10% off the face value of the card. If I need "more" Disney money, I use my Target red card to buy Disney gift cards at Target; this gives me a 5% discount on the face value of the gift card.
CUTTING COSTS:
Offsite vs onsite hotels. This depends on the travel party and location. When I go solo, I always stay at AllStars. As much as I hate using Disney buses, the cost of renting a car for one person makes the buses more appealing. I am traveling solo, not going anywhere else but Disney, so I do Mears Connect from the airport ($32 round trip) and use the Disney bus/transportation system. One or two people in a small hotel room in manageable for me, but 4 or more is not. That's when offsite becomes more attractive but you have to either adjust your schedule to meet a hotel's bus-to-Disney schedule, or you need to (a) rent a car and (b) pay to park at the theme parks. You also have to pay for gas and tolls, but to me, this isn't a big enough additional cost to worry about.
Park tickets. If you aren't tying your park tickets to a room/package deal, consider the non-Disney providers. They are sanctioned by Disney to sell Disney tickets, usually at a small savings. TouringPlans has an option where you tell them the kind of tickets you want/need and they will tell you what the costs are from the different ticket dealers. I find this the easiest way to find the cheapest tickets (right now, if I'm purchasing tickets with cash for our upcoming trip, I can save $151 on five tickets thru one option I found on TouringPlans. That's more than half of one dinner for the 5 of us, give or take, depending on where we are eating).
Food. I/we order from Kroger when we stay onsite. They will deliver directly to your resort for a minimal fee ($3-$10) depending on the time of day you want the delivery. We order our breakfast and snack stuff (that we can't or don't want to pack)- coffee, half-n-half, fruit, cereal, cheese, crackers, iced tea, seltzer, bottled water (usually by the gallon, and pour it into smaller bottles to take into the parks), maybe some snacky-things. Usually we don't get junk food for the room as I know we are going to have an in-park snack during the day, and we try to limit our sugar/junk intake. If we do buy snacks, it's the crunchy/salty variety, not sweets. We ALWAYS eat breakfast in the room (unless we have decided that our sit-down meal for the day will be a character breakfast). It's more cost-effective, it's somewhat healthier than what Disney offers for breakfast in the food courts, and it saves time, eating while getting ready to go out for the day. SPEAKING OF CHARACTER BREAKFASTS they cost a WHOLE lot of money, very very expensive for the food you get. However, at Disney, time is money. If you know you are going to have to stand in multiple character lines in the parks, let's say 45 minutes at each, that's a lot of park time to "waste" standing in line when you can buy the same experience by doing a character breakfast. You spend an hour eating (gotta eat anyhow) and get to visit with 3-6 characters at the same time, instead of standing in multiple lines in the park during the day. it's a trade off, it's "disney math" at its finest, but we always saw the value in getting the characters out of the way while sitting in air conditioning and eating at the same time, even if we had to pay for the experience.
We could take sandwiches/lunch food into the parks to save money but have found that we need an hour sit-down in the a/c during the day. To save money here, we don't buy desserts (because park-snacks are planned), drink water with our meals, and sometimes share meals or get a kids meal. None of us eat as much in the heat of Florida, so this works, and... see the aforementioned park snacks! Nobody ever goes hungry when we are at Disney, but we also don't get big lunches with lots of extras. Among other things, if you have lunch leftovers, there is no safe spot for saving them (unless you have a car with an ice pack filled cooler in the trunk, and then you have to schlep the leftovers out TO the car).
Dinner- or the "big meal of the day" regardless of when it's eaten. This is where our money goes. We are on vacation and we LIKE to go out to dinner, LIKE to eat at restaurants that are different from the offerings in our home area. However, the lunch rules of thumb play here, too. No sodas, no dessert, usually no apps. Most Disney restaurants provide good-sized portions, enough to be satisfying. We will do an app and split a dinner, or do several apps to share instead of entrees, etc. And, yes, the 'rules' break down sometimes, like when the dessert menu is irresistible and we can share, etc.
ALSO you can get free ice water from any Disney counter/quick service place in any of the parks. Just go up to the counter and ask for some ice water and they'll give you a cup. Easy, free way to stay hydrated without having to carry your own heavy water bottles, or pay Disney prices for a bottle of water.
ALSO #2. We skip the dining plan. If you don't drink alcohol, or have kids in the 9-21 age range, the dining plan is a terrible deal as it includes the "price" of one alcoholic drink. We DO drink, but we don't eat the way the dining plan 'wants' us to, so we never get any value here. I have kept receipts for meals and snacks, and it's always several hundred dollars cheaper for us to NOT do the dining plan. if you like the pre-paid aspect of the dining plan (so you don't order dinner by price rather than what you want to eat), do a ball park estimate of what you anticipate spending on food and buy a Disney gift card in that amount, specifically for meals. If you have leftover money on the card, buy something to take home... snacks, small souvenir... or save it for next time. Don't forget to figure in the cost of tax, and do all your tipping in cash so the card "lasts" for the duration of the trip.
SOUVENIRS. This is a biggie. We don't usually "do" souvenirs... we'd rather have nice meals and save the money for the next trip. Having said that, we also usually let kids pick out one thing but there is a price limit, and it's not very high, maybe $25 per kid. We avoid that most times, though. I have been known to buy Disney shirts from Etsy before going on a trip... not a family matchy-matchy shirt, but just a shirt that I know each kid would like, different shirts for different kids, and give them to them on the first night in the hotel. They can decide when to wear them. One thing we ALWAYS bring with us is glow sticks from Dollar Tree, enough for the kids to use as many as they want each night. It's so freaking cheap, and we can get enough glow sticks for EVERY night of the trip and have money left in our pockets instead of buying one stupid light up toy.
ANYHOW... this is how we did/do Disney, to save money.