1) Buy and use decorations that can be relevant for various occasions
For example, I have three table clothes (in red, golden yellow and blue) that can be used for various celebrations (Valentine's Day, Christmas, Birthday parties, Fourth of July, etc.). By keeping to a versatile color scheme, you save on money and time (don't have to shop for new decorations each time you have a planned party).
2) Use the chinaware not paper plates
For some reason, some people equate casual parties with paper plates usage. Break this trend and use your dinnerware. The amount of time it takes you to toss the bags of garbage filled with paper plates, you would have already loaded your dishwasher with the dinnerware. Also, when paper plates are available, individuals tend to use new ones for second servings and generate even more waste. For some reason, individuals are able to better keep track of actual dinnerware.
3) Have a dozen permanent markers available for marking up the plastic disposable cups so individuals are less likely to use new ones. For your more sophisticated parties, consider using actual glasses.
4) Lower your thermostat
When there are more people at a home or single location, the crowd generates heat so you can lower your thermostat and people will not notice the five degree difference but you will notice it in your utility bill.
5) Use actual household plants for centerpieces instead of fresh cut flowers or consider displaying various bowls of fruit.
6) Designate a recycle garbage can and label it accordingly. Instead of having possible recyclable bottles and other recyclable items tossed out, a simple designated recycle garbage can will enable guests to clean up and recycle responsibly.
7) Forget about giving away party favors and if you really feel compelled to give party favors, make them eco-friendly.
I've attended a fair number of kids' birthday parties and don't really want another bag filled with dollar bought plastic toys that break easily and are basically glorified landfill items. So, avoid giving away party favors and if you really feel compelled to give something away consider giving away one or more of these eco-friendly items: mini-bag of seeds, recycled crayons, homemade mini-journals, BPA free water bottles, recyclable lunch bags, recyclable shopping bags/totes and cloth made coin pouches.