Giraffe has some really good ideas-- I’ll definitely second getting a temporary liquor license and finding a place that will allow you to supply your own alcohol. That alone saved is more than a thousand.
Another idea for flowers is to get them at a Farmer’s Market on the day of or day before the wedding. Depending on what time of day your wedding will be, and how formal, the flowers at Farmer’s Markets are consistently beautiful and varied. It’s worth checking out, anyway.
We also did our own invitations, which turned out beautifully, and instead of the traditional cake, my mom and sister (excellent human beings that they are) made 140 Black Bottom Cups, my very favoritest cupcake in the world. We stacked them at the reception in three tiers, so it looked cake-ish. It was cool, if I do say so myself.
I’ll also second Chef Troy’s advice to double-check every bid you get from caterers. While you’re talking to them, explore your options of sit-down vs. buffet vs. food stations. By setting the food up in different ways, you get a range of options and thus a range of prices. We opted for food stations with lots of appetizers and two pastas. It was damn tasty.