Hi AJ
If you push Sonar into the red you're introducing digital distortion which sounds nasty, so always make sure you have enough headroom on your final mixdown.
There's loads of ways to get more volume, they're basically all compressors of some sort and everyone will have a favourite. I use waves L3 which is a multiband compressor and a brick wall limiter , but try not to squash the song too much.
If you want something cheap but very good , then once you get your head around it Harbal is excellent for final eq and it has it's own limiter so you can match the volume of another track. You can literally get a pretty decent result with that on it's own using the automatic eq ( intuit Q) and gain button ! Though you will get better results if you take the time to learn to use it in more detail, it can also help show you where your going wrong on unbalanced mixes or highlight monitoring problems that may be room based and you're overcompensating.
CosmicD
We never finish a mix... we simply abandon them.
You can't polish a turd, but you can always spray paint it GOLD
Great songs are not written, they are re-witten