Well, for us, we look to buy cheaper airfare (although once it's booked, that's a fixed expenses) I also look for the cheapest car rental I can. I do the same for park passes.
Then you have to decide where you are staying. generally, for an equivalent sized room and amenities, off site is cheaper than on site. We have stayed both, and while I loved on site at DxL/PO, I needed the space at our timeshare. That's a decision you'll have to make, but it's once spot where you can make significant changes to your budget.
Once those are done, here are my others. We usually eat b'fast in our room (hotel or timeshare). I keep bagels/cream cheese, oatmeal, hot cocoa packets, fruit, cheese sticks, etc. Last time we did do characher b'fasts, but not too often. We also generally eat 2 meals a day at Disney, so b'fast in the room and then lunch and dinner, or big later b'fast and early dinner.
I bring snacks in our backpack.....generally some cheese and crackers like things, and some useless junk the kids will enjoy (like baby bottle pops, fun dips, etc....I buy at Smart and Final before we go) Snacks in the parks are rare for us, and a special treat. For our last 10 day trip, we had snacks twice....ice cream bars and popcorn. We carry water bottles in the parks, too. It sounds like we carry alot, but we don't.....either dh has one small backpack, or we each carry a fanny pack. For us, a family of five....snacks can run up alot of money, and I just don't think they are worth it. (That's just my opinion)
Also, we buy few souvenirs. While this is money saving, it's also a philosophical thing for me, so I strongly believe it. We tell our kids our vacation is not about shopping, it's about being together as a family. It's about doing fun stuff and getting to spend special time together. My boys spend special time with dad doing "scary" rides, and dd does shopping with me (but she knows I like to window ship, and don't want more "stuff") So, if we buy a souvenir, it's usually something special. My oldest ds bought a really nice Indiana Jones hat (about $50) we paid half and so did he. DD got a beautifal Jasmine costume (again we paid half)
So, snacks and souvenris save us money, too.
For meals, counter service and off site will save you money over nicer sit down meals. Having said that, we generally eat on site, and have one counter service and one sit down meal each day.
Truthfully, it's all about your priorities....what are "musts" for you, and what you can live without. I like to take two or three vacations a year (not Disney), so I won't spend alot of my vacation budget on upscale lodging. I don't need more "stuff", so spend little money on souvenirs, and snacks are overpriced (IMO) so I'd rather bring my own. But, I travel at busy season and value a PS in a restaurant to take me away from the crowds and crziness. Our last trip, the kids did the Pirate Cruise, and we did HDDR and took an Illuminations Cruise. (because of the fun time)
Hope this helps.