Alright, so I've seen a couple of little DIY's on how to make your filter better, but they all seem very confusing and hard to understand, so I'll try my hand at writing one
This tutorial is written using an Aqueon (formerly "All Glass Aquariums" I think) filter, as you can see in the picture
Normally, your filter will contain some sort of cartridges, for this one, there's a tall rectangular cartridge filled with all of the components (carbon, ammonia remover...) it's that tall thing with the blue in front.
With most filters, you'll be able to find some, or in this case, A LOT of space behind, beside and all around the filter cartridge, BIG EMPTY SPACE, as you can see.
I thought about putting a small heater inside the deep side part but didn't want to spend the money at the time (also my local Big Al's was out). You could do that or put in an airstone, anything that the big space could be good for.
I was lucky enough to find some large filter cartridges from another type of filter at Big Al's for a discounted price (overstocked I think). They fit PERFECTLY into the side as well as the back of the filter, but i could probably fit some more, or even some filter floss or sponge to help bacteria growth.
Instead of a sponge I added something called Matrix (by Seachem). It's kinda like BioBalls except small stones with millions of air pockets, they say that they're more efficient, and also, the smaller size worked for my filter.
Another good idea would be to take a filter sponge, cut down the center of it and attach it to the filter intake. You'd need to make sure it was open-pore and that it was cleaned often, since it could clog pretty fast. I chose not to since my filter fits into my DIY'd background.
These ideas can be applied to almost any filter so give it a try. I've found it to help quite a bit!
Chris