Dear shauntay, to my knowledge there is no cream or pill that will keep your breast firm throughout time or pregnancies. They may try to sell you a few but I don't think they have an apreciable effect, you'll be throwing your money away.
What I do know is that there are three things that will impact your breasts negatively and they are (in order of severity) Pregnancy: The hormonal changes of pregnancy will make your breasts engorged, that is larger and fuller, while you are pregnant. After birth and if you choose to lactate your child (which I highly recommend) this engorgement will slowly go away and your breasts will reduce their size to normal or less. How long you lactate your baby will also affect your breasts. In my experience lactations of a year or more almost always cause severe damage to the breasts. Now a days people don't have the leisure of extended maternity leave so lactations are stopped after a few weeks or months with minimal effect on the breasts. No lactation also protects the breast but you and your baby will miss out on a wonderful experience.
The second thing is weight fluctuations (they certainly happen during pregnancy) The typical example of that is yo-yo diets. I see many patients that lost massive amounts of weight and they always have major breast firmness issues.
Finally aging certainly has an effect in the quality of the breast skin and breast tissue but much less than the other two. But if you add all three... the effects can be devastating.
Not everybody is affected the same way, genetics certainly plays a big role in the final outcome. I hope I didn't scare you too much with this reality but I think the best cream or pill will be to maintain your weight at an optimal level especially during pregnancies and keep your lactation time under 6 months, take good care of your skin and your breasts will last you well into old age.