Soundproof your environment as best you can. Use filler material, such as blankets and cushions, to cover areas of the room where sound might leak in from outside. This will also help absorb the sound of the vocalist so that the recording is dry with less reverberation.
Use a dynamic microphone if you are in a noisy environment. Though ultra clean studios may prefer a condenser microphone for its warmer sound, a dynamic microphone is less likely to pick up unwanted background sounds.
Use a pop filter, always. A pop filter is a thin screen that is placed between the vocalist and the microphone. This will reduce pops and clicks in the recording and make a cleaner sound recording.
Instruct your vocalist on proper posture. Have him or her stand straight and sing directly into the microphone. If the vocalist is singing especially loud notes, have him or her pull back from the microphone so that the signal does not peak.
Stop recording and start over if you hear a loud, unwanted sound. Use your best judgment. It may be harder than it at first appears to later remove this disturbance.