I know that it's a cardinal rule of mixing to do a high pass/low cut on the parts, but I don't always do that (although I really should).

Although I'd like to take credit for things I'm doing in the mix, I suspect the main thing you're hearing is the BuzzRizer free plugin I'm using. It's a multiband compressor, which means you can apply different compression settings to different frequencies. I'm using the "Classic 3" setting.

Bombardier is also quite nice, but not free. You can download a fully-functional evaluation copy if you'd like to try it out. It also adds a lot of clarity to the final mix.

Toontrack EZmix also has a nice "Master" preset that you might want to try. It's also non-free, but you can download a time-limited demo (15 minutes per session).

I'm sure there are lots of other nice tools that are available, but those are the ones I know about.

That said, I'm pretty careful when setting up a mix that not too much is going on at a time. I automate the faders a lot, bringing instruments in and out of the mix and try to make sure there's only one instrument that's got the primary focus. I also try to keep the instruments as low as possible, so there's not a lot of fighting in the mix.