How important is "unobtrusively"? If the artist doesn't care, I wouldn't care too much. I've never had a complaint from the audience.
Recording from this past Saturday:
http://www.youtube.com/watch?v=61DgL-uij1057 with windscreen for the vocal, 57 and 545* for the guitar, two widely spaced (I think about 3'-4') AT853s at the back of the room for audience/ambience. The only real "trick" I'd say (for more flexibility later) is to put the guitar mic higher and angle it down, and do the opposite for the vocal mic. It doesn't affect the intended sound from each mic much, but it greatly increases rejection of the other sound (guitar sound into the vocal mic and vice versa). The stands in front are just standard On Stage Stands boom stands. In my case I mounted the AT853s above the opening to the dining room using some 3M Command hooks and some small bits of wire.
Of course, if you're only recording on 2 tracks, that won't work, unless you do a live mix...
Once (before I had all of the gear I do now) I did a recording with the AT853s spaced maybe 9" apart and angled up towards the artists. It sounded pretty nice, except the guitar tended to overpower the vocal due to the guitar being that much closer to the mics. If you're doing that type of setup you'd really want the mics more up at the level of the singer's mouth. But I haven't experimented at all with that, since I haven't had to. And of course if you're just using 2 mics it's more important to get a nice balance, as there's no way to change it after the fact (unlike with my setup, where I can I can adjust the balance of the close and room mics later).
*Normally I'd just use a single guitar mic, but I was setup for another performer who ended up having to leave early, so I moved that guitar mic over; I thought the two sounded nice in stereo. Otherwise I usually pan the (single) guitar mic left. The 58 in the video is unused.
I've had a 6 piece band twice (well, 5 of them were there the first time), amplified vocals, instrument amps...that was something totally different. Ended up with a very nice 8-track recording.