Beepster
Cool, mang. Honestly my brain is on fire with annoying bureaucratic bullpizzle at the moment so it was cathartic unleashing all that. I'm just glad you found it helpful.
 
Cheers. ;-)

Definitely. Whatever helps put out the brain fire. :)
 
When I'm doing the vocals, I usually mute everything except drums, bass and (if there) an acoustic track, and I do tend to separate certain instrumentation types to their own respective buses.
 
The "freezing" aspect of tracks is relatively new to me, though, so that was especially helpful. I'm sure there are probably more things I can do within Scarlett's Mix Control, but I find those routing options fairly confusing, tbh, which probably speaks more to my lack of knowledge. I don't get what options like "DAW 6 and 7" are, for example, or what that corresponds to within "Sonar." "Monitor 1 and 2" and "DAW 1 and 2" I get, but not sure about anything beyond that.

Well to be honest even though the MixControl stuff is supposed to be "easy" to configure it always confused me and I found other ways to get things done anyway. It mostly seems more useful for if you need to send different mixes to different band members while they track simultaneously.
 
The only times I open it are to check input levels (because I find even if Sonar isn't clipping the MixControl may be) and to change my ASIO buffer settings.
 
I will say that, if you don't already, it is EXTREMELY helpful to have busses for ALL instruments. Even if you only have one bass track or one guitar track or whatever having those routed to a bus makes so many things so much easier. I never used busses before I got Sonar (didn't even really know what the heck they were) and good lord did it ever make things unnecessarily difficult.

I own the Scarlett 6i6 so might be able to help you. 
At first I did have trouble figuring out the mix control and I was seeing clipping of the meters inside the mix control GUI that freaked me out. The mix controls levels have absolutely no bearing on your record level. They are for monitoring only. And they seem to show output and input clipping even though you will not hear it. 
 
I just recorded a vocal a few minutes ago. 
He's the drift. 
 
So it's very simple to have your mike roast your ear drums and turn the music down to the exact level. 
 
You say your using a good condenser mike and it seems to clip sometimes. Is that in Sonars track meter?? Because that's the only place it matters. 
You might be turning your mike down way to far and if you go by the mix control meters you will be mislead. 
Anyway, in the mix control, turn the fader for your mike channel on full...pin it!! and ignor the red. I found out it has no effect on the recording, turn it off and your level in Sonar will stay the same. It is the level control on the front panel that adjusts the input level. 
Now turn up the headphones until it's where you want it, for me this is about 3 O clock.  
Now turn up the DAW/ MON 1-2 output ( second pair from the right) until Sonars playback is just right. 
 
If you are still having to turn the mike down below a normal level then I would either practice mike technique or buy a mixer or pre amp and a compressor. 
 
 

Oh and Beepster, there's a huge difference between the entry level Scarletts and the Firewire models as far as RTL performance goes.    
 
The 2i2 -2i4 6i6 ( 18i4?)  have unacceptable round trip latency for real time monitoring. I have about 32 ms RTL.  But I don't use the zero latency setting either. I'm not sure what difference it makes. I'm using what ever the dang thing defaulted to when I installed it and rarely pay attention to it unless I'm fussing with the mike input headphone thing. 
 
Bottom line is, unless you toggle the input echo in Sonar and listen to what's passed though all those converters and stuff, your using zero latency monitoring because your headphones / monitors are patched directly to the inputs. If you turn on input echo in Sonar you will hear a delay. All interfaces work this way. 
 

I don't see any reason for a 'headphone amplifier'. You already have them in your Scarlett and they should be pretty good (sorry I realise you have made a purchase).

Within mix control you turn up the volume of the mic, and if that isn't enough turn down the volume of what is coming out the DAW (Sonar). In addition there is an overall volume.

In Sonar make sure the master bus volume is set at the default position and forget it.

BTW If you are hearing your voice through the DAW in mix control whilst recording then you are not using the zero latency features, echo in the Sonar vox track should be muted. If there is any reverb required route it to a bus and make sure it is 100% wet. You don't want to hear the dry vocal coming out the DAW unless you are playing back. When playing back you can mute the mic in mix control.

So you've done this and volume fluctuates too much? Check the levels you recorded first, is there really a massive change? If that's unacceptable you need a good preamp/compressor after the mic and/or a more suitable mic.

Personally I don't bother with zero latency monitoring any more. Too fiddly and I find the results without it very acceptable with my Focusrite Saffire, and you can a VST compressor into the chain (no need to fork out for a real compressor).
 
[Edited to cross out an incorrect statement]

Some great advice here.
 
I have one other suggestion.
 
 
SING LOUDER!!!!!!!

Here is a very useful mix control video, once you understand the inside/out you are empowered and will realise just how brilliant and flexible it is. It will no longer be 'fiddly' and you will wonder why other audio interfaces don't work this way.

http://youtu.be/M1avPHhiGw8

Kerch
...I have a set of headphones...

You haven't said which headphones you are using. I'm just wondering if maybe it's the headphones that are the problem.

Humm. Dont have this problem either with the Scarlett. There is some good advice already, particularly about the levels. Just make sure your not clipping in the track within Sonar. I use an outboard compressor/pre amp to get my level at best, and still go through the Scarlett mic amp, keeping my record level as close to -3db as possible.
 
To get ample volume from the microphone/headphone for both me and the vocalist, before I start, I bounce a track with a mix of what I want the vocalist to hear. I find I usually only want the basics like bass/drums/rythym guitars etc. No lead, distorted guitars etc. But that varies song to song/ singer to singer. Once I have my bounced track for the vocalist to follow, I mute everything else, then it's just one fader to control the music. I find I have that down somewhere around the -20db mark or so, which allows you to turn the headphone volume right up, usually to around the 3 oclock mark like someone else said, which is quite loud. Any louder is simply too much I find, and will tire not only me, but the vocalist very quickly. The Scarlett has heaps of headphone volume onboard, so I agree, you do not need a headphone amp. Having said that, I also usually use a headphone amp for other's in the room to monitor what's going on, so you will find a use for your new purchase.
 
Hope this helps.
 
Dave

Cakealex - one thing- 
 
"and you can a VST compressor into the chain (no need to fork out for a real compressor)." 
 
This is not true, You cannot compress before you hit the A/D with a VST compressor which is the why some of us work this way in the first place. It is one of those topics that can start a long discussion because it falls under the category of "technique". But there's a goodly number of engineers who will put a compressor on the pre amp to catch any overs. Once an over has hit your A/D your hooped. The argument will always go, well then just turn it down...but myself I have worked this way since the 4 track tape deck days and it works for me. A fast attack and a very low ratio will catch explosives and bad mike technique. And without actually compressing the vocal. 
 
People get confused about the Zero Latency monitoring and there was just a thread about it. Simply put, all interfaces have a way to listen to your input either in the cans or up on the monitors. You then blend in the output of the DAW  and go at it. All DAWS adjust the playback so that overdubs are in sync. That's why your DAW needs to calculate the latency. 
Everyone should take the time to test it's accuracy. 
 
Ya it's to bad the OP ordered the Headphone Amp before waiting for a few more replies. Not a bad thing to have as they are handy for recording bands and groups. The Scarlett has 2 (loud) Headphone amps built in but you would want the outboard unit for using the rear outputs for a cue mix.