The secret to great fitting clothes: a tailor. Anyone who has ever watched a makeover show has likely heard this mantra. And it's true—besides making your clothes fit better, working with a tailor gives you a better idea of what clothing works best for your body and can even help ensure your clothes last longer.

But while tailors are often lifesavers, they're not miracle workers. Below, we talked to Diana Spring Sidley, the owner and seamstress behind Mentionables in Cleveland. She told us what you need to know before bringing in your next dress or pair of pants for a total rehaul.

Rule 1: (Most) clothing cannot be let out more than 1"

The 1" rule applies to most garments—but some items can't be taken out at all.

Whenever someone contacts Spring Sidley with the request to take something out, she says the same thing: "I have to see the garment."

From garment to garment, the amount of fabric left inside varies. In general, formalwear is far more likely to have a significant amount of fabric available for letting out. In some cases, Spring Sidley has let garments out more than an inch by finding matching fabric and adding panels, but this is a lot more work and won't always give you the exact result you're hoping for.

Rule 2: Don't have a dress taken in six sizes

Although many items might only be let out an inch, Spring Sidley has taken garments in up to six sizes. But just because there's plenty of fabric to work with doesn't mean the job is any easier. Once she worked on a bridesmaid's dress that was so large, its form was unrecognizable.

Rule 3: Don't measure yourself with carpenter's tape

That huge bridesmaid's dress mentioned above? Yeah, the reason the sizing was so off to begin with was that the bridesmaid "actually measured herself with a carpenter's metal tape measure, so the measurements did not turn out terribly accurate," Spring Sidley said. "It was such a sack on her that I had to have her pull up a photo of what the dress was supposed to look like. I couldn't tell from looking at her how it was supposed to fit."

Tailor's tape is called tailor's tape for a reason—it's flexible enough to conform to the body and get an accurate measurement. And getting accurate numbers is crucial if you want something to fit correctly.

Rule 4: Keep the changes simple

Some types of clothing alteration, like hemming pants or taking in a suit coat are pretty straightforward and yield dependably good results. But while seamstresses and tailors work minor miracles with garments every day, some requests are too much even for them.

Spring Sibley has had some women bring in an evening gown and request dress alterations to make it an entirely different dress. While not always impossible, it's sometimes hard for a tailor or seamstress to really know if a client's request is possible before they start working. "You never know what it's going to look like inside until you open it up," she said. "I've done sets of bridesmaids, and I open [the dresses] up and they're built differently."

Because of this, she finds menswear to be the easiest type of clothing to work with. "From suit to suit, it's pretty much exactly the same on the inside. It's the same process," she said. "They pretty much all have the same rule of where the hem should lay, how the suit should fit."

Bottom line: your seamstress can do a lot, but don't be surprised if the finished product doesn't look exactly the way you pictured.

Rule 5: Don't alter a garment more than once

Although the same garment can be taken in and let out repeatedly in theory, one problem often presents itself. When you let out fabric, you can see where it used to be sewn. On durable fabrics, such as cotton and linen, this isn't as noticeable. However, it's much more evident on delicate fabrics like chiffon or taffeta. "You really kind of have one shot at sewing that seam," Spring Sidley said. So more than one round of alterations is out of the question. 

This article was originally published in 2014 and has since been modified by our editors.