I was having the same trouble as you, until I found this blog post:
Configuring rEFInd boot manager as your startup disk on a Mac
The author, DavidJB.com, describes how you can set the default startup disk by pressing the control key at the same time as selecting the startup disk from the Startup Manager. (You need to hold down the option key during the boot process to invoke to the Startup Manager). The author suggests holding the control key when clicking the disk, but I didn't have a pointer, so just held the control key when I pressed the return key after selecting the desired disk.
So far, it has taken me straight to rEFInd, which I have installed on a separate HFS+ partition, every time I've restarted since then.