I personally use a medical tape for mine, when I need a cleavage, it is called 3m transpore tape and it comes in 1/2 inch (which is great for lifting eyebrows invisibly) and inch strips which I use to generate a cleavage. it is as sticky as Duck tape on first contact, but remove much simpler and cleaner. it only nice for makes use of but you will find it better. I do much the same as you do, but one time taped I use a silicone breast form to push the cleavage over, Much safer than water balloons, but others here use silicone chicken fillets or bird seed sacks.
I use oval plastic discs, stuck the the side of the breast, tape them together, and then push my breast forms in to the sides moving the breast over giving a actual cleavage affect. (I used to make use of medfix, but transpore is much better for the job)
Here is another made with Platic Bucket.
It is cut out of a plastic bucket, two pieces that fit snugly around the outer rim of your breast tissue. A couple of strings to tie it around your chest, to be tightened and the breast tissue (everyone has some some tissue that can be used tof the purpose)ï¿½pulled towards the center as much as possible to create the cleavage. Wear a bra and forms over it normally.