FItting breast forms can be a little confusing for new wearers - here are some tips:
With breast forms the most important thing is the bra. Every company has a different size and shape and it varies so much between styles.
Low cut balconette bras will always struggle as they just do not have the coverage at the front to support the forms so try to avoid those if you can. They aren't as sexy to some, but the more support you can get at the front the better the breast forms will be held in.
You also really need to make sure you are wearing the right size bra. If you look side on, the curve of the cup and underwire should follow the forms perfectly at the side under your arm. They should be pulled tightly against you by the bra and should not pull away from you at the front between the cups. The band around you should be very tight and feel too tight. It takes time to get used to it, but you should only just be able to get two fingers in there between your bra and your skin (this goes for women as well, and it what most women do wrong). Then follow the cup up and it should hug the forms, with no gape anywhere, or very little at least. The straps should then be pulled tightly on the shoulder to take the control at the top of the bra. They should also be nice and tight so it all feels secure.
The absolute ideal is to take them bra shopping so that you can ensure that the bra fits well in a changing room, but that's the bit that's often hard for TVs. If you can though, then go for it. If not order a few bras, maybe from different companies and then send back ones that don't fit. I buy my bras from La Senza and I find that some styles fit me a lot more than others, so it really is worth trying to make sure you get the right fit. If you ensure you get the right fit, the breast forms/false breasts will feel snug, hugged and part of you.