If you live in a high rise, use the party room. If you don't, ask a friend if you can use theirs.
Make cupcakes instead of a cake that you have to cut. This will save so much time and energy at the party it isn't even funny. Also, buy the cake mixes and frosting at Aldi. 79 cents for the cake mix and 99 cents for the frosting. I have been using their mixes for about 20 years now and people always rave about the taste. This will save a ton of money!
When you buy the paper goods, go to a dollar store, not a party store! Although it is tempting, just buy one or 2 of the party themed paper goods. For example, buy the party napkins and a couple of reusable party favors, like a few party tablecloths and centerpieces. This will set the mood for the party without breaking the bank. Don't buy stuff that will be thrown away and be sure to carefully pack away the stuff that is reusable. I have a door hanging that says "The Party's Here" that I have used for years. The trick is to make sure you tell your helpers to carefully put these reusable things away and not to just rip the door cover off the door and tear it. Walgreens has the cheapest paper plates around.
If you are buying party favors for the kids attending, buy them each one thing that will last. Forget the cute cheap plastic toys that mom will throw away as soon as they get stepped on.
Again, the dollar store is great for this. You can often find kids books or stuffed animals for a buck. If you buy the stuffed animals, be sure there are no eyes or noses that can be chewed off by the babys.