So I've noticed that I can occasionally get nicer results- louder, more expressive and aggressive- vocals when sitting down hold my dynamic in my hand, keyoard below my right hand, rather than setting up my makeshift soundproofing and setting my mic on my stand, five ft away from my keyboard and trying to give it a shot.
In addition, I never NEED the treatment when sitting+handheld!
I imagine standing SHOULD be better, but at the moment I'm not totally convinced. Any solutions?

Standing is better for projection because your diaphragm isn't compressed. However, you may be hearing better results not because of your position/posture but because of a shift in room acoustics.
 
Comb filtering is the biggest problem when recording vocals. It's caused by reflections off walls, floor and ceiling - generally, the closer a reflective surface is, the worse the problem. People often surround themselves with absorption to counter the effect, but often forget that the closest boundary is usually the ceiling, especially if you're standing. Hanging some rigid fiberglass from the ceiling is the solution for that.
 
As for hand-held versus stand-mounted microphones, I do like to hold the mic in my hand, as it's much more natural when trying to keep levels steady. However, it's not an option with most condenser microphones because they're subject to picking up handling noise. For a dynamic mic, though, hand-held is the way to go, IMO.

Either works...standing or sitting.... depending on the song mostly. And really, I'm in the camp that most of the time it doesn't matter much.
 
Having said that, I think it's possible to sing slightly better standing. As Dave said.... you can dig down deep for the power and projection while standing.
 
I like to hold the mic.... condenser.... if it's a quick vocal dub.... kinda hard to do that for the guitar though.... so a stand works best. 95% of the time I use a stand... the condenser gets a bit heavy after a while.
 
You'll get a kick outta this>>>  http://www.youtube.com/watch?v=XlNHrbyeCxE 
 
yeah, I hold the mic every now and then.

If you play a keyboard (synth not computer keyboard) then if you sit and play, then you usually sit and sing and play also. If you play guitar, then you stand and sing, and if you play drums then you have to sit and sing and play the drums. If you play bass guitar, then you probably stand and sing, unless you are drunk and then you may want to sit down and sing (if possible) before you fall down. If any other instruments, then probably the same rules apply, and if you are jumping up and down while playing a song then you better have a sturdy floor, as long as the house is rocking with you or wherever you are playing except at a funeral.

 
 

Which ever makes you most comfortable and gives you your best performance.  That goes for sitting/standing and also hand held.  But I will add most vocals are recorded w/ a LCD and stand while the artist is standing.  It is something you should work on getting comfortable doing.
 
@

I sit (to get at the mouse and all) and use a boom stand for my vocal mic.  If I was recording a singer, they would stand.
 
Rimshot

A rumor I heard when I was a kid that Mike Love sang most of his songs on the "Holland" album while lying on his back.

When I worked at Capital Records in the 80's as a studio drummer, their chief engineer Charles Ferris recorded Glen Campbell's lead vocals with him in a long padded box on his back with a mic above his face with the lights out.  Almost like a coffin.  Whatever works!
 
Jimmy
 

I usually stand, but depending on the song, and the delivery required, I may sit sometimes as well. To me it is dictated more by the sound you're going for.

My vocal teacher, a professional opera singer, taught me that you should stand.  Not only to keep the diaphragm free, but also to align everything above it.  Throat, neck, chest, etc...  Kinda like the curves in a brass instrument, they're meant to be in a certain position for optimal sound shape and presentation/projection.
 
Having said that, it may not matter depending on what you're singing.