Until we all can get in shape to wear these outfits of ours, there are things like the old "girdles" that can help you look more fit and shapely. I was told that Jonathan Frakes wore one to look slim in his TNG jumper!
In the movies and tv, men use them as well as women. They aren't cure-alls. Nothing beats diet and execise to get into shape, but as a temporary or cosmetic device, these "cincher/body shapers" are ideal. Unlike the old "girdles", these devices are more comfortable and can be worn for extended periods.
If you are considering these shapers in anticipation of getting your accurate Starfleet uniform, you will need to get "before and after" measurements. Do note that even with these shapers, you will need to keep your weight constant. They're like a band-aid or a temporary fix, but they won't perform miracles! If you have a 42" waist, it won't slim you down to a size 38. If you're slightly overweight or just want to have smoother curves, these are ideal, but they won't magically transform a person from "John Goodman" into "Brad Pitt"!
Here are companies that sells various type shapers for men and women:
These are just a few. You may even find these items locally as well.