Insert ReaEQ on master track, HP at like 700, LP at 2k or so. Distort if you like.
Click the "Env" button on the master track.
Locate a checkbox that says "WET". Click that check box. A lane for automation now opens under the master track in the media bay (where all your tracks are and stuff).
Insert a couple points and draw an automation curve from 100% wet to 0% wet.
Should sound like this https://dl.dropbox.com/u/35204107/Lofi_fade_in.mp3
I wouldn't automate mix knob but frequencies of each band to ope sounds.
When I do this stuff I probably add overdrive on master track and automate mix on it after it add EQ with Hi-pass somewhere 700 and experiment with resonance (if add resonance that automate that too to take it off) + lowpass around 2k and also try resonance (if I remember last time I put there resonance more than on HP)
There is my example
[SOUNDCLOUD]http://www.soundcloud.com/nezversstudio/we-call-this-irony-300-nezvers[/SOUNDCLOUD]
I don't like the way the frequencies "sweep" under outward automation curve. An audible "flange" just feels wrong to me.