Nearly all the tailors mentioned here (Sam WW Chan, Asha etc.) will do a decent job provided you know what you want.

1. I normally take along a shirt and suit that fits me well.
2. I pick the fabric;

3. Carry a photograph of the style I want or choose from a catalogue that the tailors normally have.

This way I minimize surprises - Do keep in mind, that a minimum to two fittings for suits are an indication of the integrity of the tailoring shop. Be suspicious of no fitting for a suit.