No lube/soap in the hot water. Added it when I was just about to mount them. One key thing I did was to take a large regular pliers, keeping the jaws closed, inserted the "head" of the pliers into the grip so the grip stretched over them. Then I opened the jaws to stretch them as much as possible to loosen them up. Reheat in the water several times, twist 90 degrees, and repeat the stretch. Rinse repeat.
Then at some point when the pre-stretch seemed good I started to fold back the grip on it self so that is is turned inside out. This then is subjected to the stretching but using a slightly smaller pliers. Reheat as needed. Once the grip is about 1/3 folded back on itself, stretched, and heated for the last time. Then a quick lube, and go go go, before the rubber cools too much. Once the inside of the grip is in place then unfold the rolled over section in to place.
This way you are not tugging on the grip with tools that will tear the rubber. Good luck.
F800GS Mag/Black 2009 "Faucon GriS"
Farkles; Garmin GPSMap478 in a TT cradle, Airhawk seatpad, Pivot Pegs, SW-motech bars, MOD skid plate, Altrider headlight guard, HDB handguards, Maier Mudguard.