Sorry I just saw this thread and there have been some good suggestions already posted but let me add my 37 years of building custom ENERGY STAR Platinum rank homes experience to the mix…

First, many utilities offer a free energy assessment of your home where they will conduct an energy audit and usually a blower door test as well as a thorough assessment of your HVAC system. Having that information will allow you to prioritize where you can bet the best bang for your buck. As others have said adding insulation is one of the best ways but not necessarily THE best way in your particular situation.

Mini splits can help but once again some adjustments to your current system may prove more cost effective.

What kind of foundation do you have? If it’s a crawl space with your HVAC unit down there you might want to convert it from a vented to a non vented crawlspace.

There are several other options but getting the free energy audit should be your first step. If for some reason your local utility doesn’t offer one you can find contractors on the ENERGY STAR website.

Good luck!