Dress up to get dressed.
When you visit a tailor, you should wear dress shoes – don't go barging in there with some limited-edition sneakers. When you try on your trousers, you want to see exactly how they fall on what you would normally wear with a suit. The same is true for shirts – wear a dress shirt. You want to see how the sleeves and collar look under your jacket.
If you have a suit you like, wear it to your first appointment, even if it's not a custom suit. It will help the tailor get an idea of both what you like and how you hold yourself when properly attired.