Making different volume takes the same

Use the volume automation feature. If your new to compression, that would be the easiest.

It has taken me quite a while to grasp the correct concepts needed to be able to use compression effectively.

The amazing thing is that I rarely touch and adjust the volume or level controls on my interface or anything else in the studio.  I do play with the POD2 levels.
 
Compression is not the best option.  It has it's uses but getting to know how to set you levels to be consistent is more important to getting the levels the same. 
 
If you are working an a song and think that you might need to come back to it later, I would suggest using a notebook. Put each new song project on a new clean page and use as many pages as you need to make notes on the various FX, VST's, mic levels, amp levels, etc.... so that if you do have to come back and do a new take a few days later, you can quickly dial in the settings you were using.
 
I do not follow my own advice on this and have in the past, found that I could not figure out the settings I used to get a certain guitar tone.  In that case, I had to redo the entire track to make things consistent. Lots of work, but, I also have the belief that if I played it once, I can always play it again and often better the second, third, or twentieth time through. 
 
But yeah, keep notes at least until you get a certain consistency about what you're doing in regard to levels.

Lots of good suggestions. Another option is to use the gain command:
4. If you want to switch the two channels, reverse all four sliders from their present positions.
 
 

I mostly use clip gain automation to even out, say loud and soft parts of a vocal track for instance. The clip gain is pre f/x bin so then you can use a compressor for the final touches.