I love my roombas, I have one on each floor. They don't take the place of a regular run of the vacuum, but they do make it a once a week thing instead of a once every other day. But after 5 to 8 runs, the roomba filters are pretty much shot and even blowing them out doesn't restore them back to brand new. You can of course buy replacement filters, but they cost in the neighborhood of $12 a piece for the aerovac variety. This is a 5 minute solution that will let you refurbish your roomba filters for about 12 cents a pop and gain a fair bit of performance out of the deal. You'll need
A straight screwdriver
an exacto knife or razor
some 4 x 12 register vent filters (12 of them at home depot for $4)
and of course an aerovac style roomba
The Freakduino-chibi is of course optional.

Step 1: Removing the Old Filter Material

Removing the old filter material isn't difficult, but you have to be careful that you don't break the plastic. Using a razor, first cut the tops and bottoms of each exposed filter element. Next cut the bottom of the edge filter elements on either side.

Step 2: Removing the Old Material Part 2

Pull the old material out of the spaces, being careful not to break the plastic. The material will tear and you'll probably have to spend some time cleaning it out. Once you are done, you should have a clean(ish) filter basket.

Step 3: Cutting the New Filter Material

The material I decided to use is this register vent filter stuff from home depot. It's about $4 for a pack of 12 and each filter can make up to 4 roomba filters or 48 filters for $4 (WIN!). So yeah, this is not iRobot authorized filter specs. It likely doesn't filter as well as the stock stuff, but I haven't had any issues with it in my 3 machines.
Take a filter out of the package and fold it in half along it's long edge. Cut that line which will leave you two 2 inch by 12 inch filters. You can cut that again along the short side leaving you four 2 inch by 6 inch filters too.

Step 4: Threading the Filter Into the Basket

The filter basket sandwiches the filter material between two plastic fingers in several spots. Gently separate those two fingers to feed the material into the basket starting with the first finger from the edge.
It's easiest to feed this first bit of material from the inside of the filter out by wrapping a bit of it around a screwdriver and feeding it through.
Once you have fed the filter through, leave a few inches on the outside.

Step 5: Threading the Filter Material Part 2

And now just repeat that process for the remaining fingers.

Step 6: Finishing Off the Filter Material

Fold the extra filter material over to the inside of the basket and cut it off. You want to leave enough material so that when the filter is replaced into the roomba, it's held down by the filter basket itself.

Step 7: Reinstall and Send Roomba on Its Way

Reinstall the filter into the aerovac, making sure that the extra filter material is being pinched by the basket, and then fire away! Just clean the filter like you normally would and when it starts to block up, just replace it with another 12 cents worth of material.