I’ve got some standing orders of 1 or 2 euros every month for bigger projects like grapheneos and kde. And for small apps that I actively use, I try to donate 5 or 10 euros now and then. It’s not much but I believe if everyone is doing their part we succeed in the long run
The annual KDE reminder works on me. I also donate yearly to thunderbird and other random foss projects that are important to me like pihole. I pay for bitwarden to support them while keeping an instance of vaultwarden running as backup.
The other thing I try to do in lieu of donating is turning on metrics for applications I use daily when the option is there and whitelisting those domains in pihole.