The object should be big enough by now to have a permanent stretch on those muscles.
Push it in further than the widest part of the object and then pull it back a little so it's stretching from the inside out, but don't let it slip out!
Hold it till it's starting to feel enough and let it slide back in. If you have a nicely formed object you don't need any hands to get it back in. Otherwise, you are allowed to push it back in.
Do this 10 times, but each time you have to hold it longer on the thickest part. Indeed, this will really stretch you out hard.