Patience. Cleanliness and some more patience. Yes they're tricky little blighters.
Personally, I put the boot onto the piston, but not into the groove at the lip. I then introduce the skirt of the boot into the groove in the caliper. I find it better not to slather too much grease in there. Friction can be your friend. Once the boot is roughly in the groove, I gyrate the piston slowly downwards to seat the boot into its groove. as the piston then passes through the boot it locates into the cylinder and then the neck of the boot will push up and pop into the piston groove and you're done. How it's supposed to be done I don't know but I have done many of these and that's the process I have adopted.