If you can record on location, try a church. Be careful in a church though, as it's possible there may be too much of an open sound in a large church. You are usually trying for a sound that is open and full, but at the same time contained.
If
you are doing overdubs and the tune is in a specific tempo use click up front, as you normally would, so you know when to come in. My recording was very free with tempo, so I used the breath intake on the previous tracks to get an idea of when to enter for the overdubs.
A Delicate Blend
The
sample a capella clip is in 4-part harmony with each part doubled 4 times; bringing the total track count to 16. I achieved a blend across the stereo field by panning each successive overdub of the same voice part further away from the center point than it's predecessor. Some of the phrases had to be faded out so the releases would all come at about the same time. (See below)
Use
some amount of reverb to supplement the natural reverb from your location. The amount you use depends on the effect you are trying to achieve. I was trying to slightly enlarge the space I recorded in. (I didn't necessarily want people to think "bathroom" while listening.) Using the natural reverb from the tile combined with the electronic reverb gave me the open yet contained sound I was looking for. Too much reverb can turn a nice vocal blend into mush.
Be
careful with compression in an a cappella recording, especially a classical recording. I used no compression on my recording at all. I found that even the smallest amount adversely affected the timbre of the voices. Instead I tamed some of the variances in volume simply through judicious use of the volume envelopes.
Try
adding an a cappella piece to your recorded catalog. You'll find that people are often attracted to that piece for it's simplicity and innocence.