I didn't read all of the previous responses, so I apologize to the extent this repeats others' suggestions:
1 - HOTEL/TRANSPORTATION: Save on hotel and/or rental car (or parking) expenses: If you are driving yourself and have the equipment and inclination, put up a tent at Fort Wilderness Campground. It's cheap, the comfort stations are clean and well-appointed, and you'll have all the perks of staying onsite, including Disney's Magical Express, for your free airport transfers. Otherwise, if it's in the budget, share a value resort room with your friends (and after you've booked, keep watching the WDW "Special Offers" web page for room discounts that could be retroactively applied). Failing that, find an inexpensive hotel that is just off-property and that has decently-rated shuttle service to/from the parks (or a vacation home rental, if you have your own vehicle(s)), and factor in extra money for airport transfers (if needed) and/or hotel parking charges or parking at the parks (if applicable) to see if going offsite is really going to save you money in the long run. (Also, consider whether the onsite perks like earlier Fastpass access and Extra Magic Hours have sufficient value that they're worth any added cost, particularly since you're visiting during a crowded time of year. For me, if I'm going to spend extra money on any area of a Disney vacation, it's this one: I'd rather pay a little more to spend less time in lines, even if I have to cut corners elsewhere to afford being onsite.)
2 - TICKETS: Purchase magic-your-way (non-hopper) tickets from an authorized reseller like Undercover Tourist, which will save you a few bucks over the gate price. You can link them to your My Disney Experience account and proceed as usual.
3 - SOUVENIRS: Don't buy souvenirs in the parks. If you want a memento of the trip, buy a clearance T-shirt from the Disney Store website or at your local Box Lunch and pack it to wear in the parks. If you plan to meet characters, bring a blank journal book (you can find them at the Dollar Store) and a Sharpie and get some autographs -- instant, super-cheap souvenir!
4 - FOOD: You don't need to spend a dime on food inside the parks if you don't want to. Pack your own food and bring a refillable water bottle with you. (In all the trips to WDW my family made during my childhood, we never bought any food in the parks. We stayed at Fort Wilderness in a pop-up camper and ate things like fresh fruit, granola bars, canned soup, Spaghetti-Os, pasta salad, peanut butter sandwiches, hot dogs, cereal, etc. -- whatever my Mom could freeze and pack in coolers for the 20+-hour drive and then stick in a tiny electric cooler in the camper after we arrived, supplemented with a little milk and bread from the Campground general store.) If you do opt to buy food in the parks, stick to counter service or some of the better-priced table service places. (Just Google around for some lists of the best-value table service restaurants; new ones get released by several bloggers every year.) If you or anyone in your party has a Disney VISA, check out which onsite restaurants offer a 10% discount when you pay with the card. (It's not much, but it's something.) If you have a car, venture offsite to "real world" restaurants with typical pricing.
5 - GENERAL ADVICE: Do you have any birthdays coming up? How about Christmas or another gifting holiday? When people ask you what you want, simply reply, "Disney gift cards, please!" You'll never miss those extra mittens and scarves you'd otherwise have received when you're able to use gift card funds to splurge on a round of Dole Whips or help pay for your park tickets.