@Zach Wright
I have a list of the cheapest solutions with the best returns.
Aerators and shower heads. Replace all bathroom aerators with 1.0 -1.2 gpm bubble flow (not less though). Kitchen with 1.5 gpm.
Showerheads from Niagara 1.5-2.0 gpm. Advertise these to your tenants as NEW fixtures, not water saving or they will complain even if they are getting better water pressure than before.
Attic insulation, this is typically the cheapest option to add insulation to your house because it is very easy to add and it is also one of the most effective. The more you add the more you will save (blow-in), don't skimp.
Keep your home at a comfortable temperature if you have cheap natural gas, you don't want tenants electric space heaters running all the time.
Last thing would be toilets, and I'd do these only if you've already done everything else I've listed since they are either super cheap (aerators) or very effective (attic insulation). If you have the very old models upgrade these to 1.2-1.6 gallon flush toilets from Glacier bay at home depot. These are the best toilets and they are very cheap. Don't believe the $300 toilet scam.