When I went through my corset-making obsession, I learned a lot of cool tricks that can be transferred to dressmaking. And this particular trick is my absolute favorite.
When making a muslin for a dress with a boned bodice, it's imperative to actually put boning into your muslin. A boned bodice generally has little to no ease. But without the boning for stability, it just looks like a too tight, wrinkly muslin--which makes it impossible to evaluate the fit. Obviously, however, sewing boning into a garment is time-consuming and often makes it difficult to alter the seams of a garment--exactly what you need to be doing in the muslin stage!
So, meet your new best friend, masking tape. Yep, you can tape lengths of spiral steel boning into your muslin for fitting purposes. (Does masking tape have different names in different countries? It's that parchment-colored tape typically used to cover molding or door frames when painting walls.)
It's perfectly fine for it to extend over the top of the bodice, though, as long as it doesn't poke into your armpit or anything awkward like that. Here I let the back boning extend past the upper edge of the bodice, just for the purposes of fitting. This way you don't have to cut the boning to length until you've figured out the fitting issues.
I'm currently working on my design, Butterick 5882, to wear to a wedding next weekend! Sorry for the mirror photo, but it's about all I could handle this weekend. The only fitting change I needed to make was removing some ease from the waistline. The taped-in boning made this super easy: I just removed the boning to take in the seams, and then stuck it back in place to re-fit. You don't even need new masking tape since it stays sticky!
Hope this sticky trick helps you out on your next party dress muslin!