I'll try and take a look at this tonight - not quite sure what it looks like so can't be much help atm...
I guess it depends on your shader. I can't remember the exact ones right now, but you'll either have something like "cC" or "c1". "c1" will add, and "cC" will act more like transparency. Its going to turn all white either way though... Your best bet is to dim down the previous frame's image before you add on the new stuff. something like:
SOLID black();
MIX dimmer(black, prevframe);
.
..
.
dimmer.amt = 0.9;
hope thats some use...