It depends on what you're currently using, how you're using it, and why. There are plenty of things from the easy but expensive (reserved costs) to the hard (rearchitecting). Depending on where you are in development and how much money you have there are plenty of things you could potentially do.
A lot of ways, but it all starts with an an analysis of where your bill is going now.
Generally, turn off idle things. Embrace elasticity. Autoscale. Look into spot fleets.