Most places provide instructions on the creation of non-pressurized portafilters for high end machines to get better espresso. I, however, had the opposite problem. My machine, a Delonghi EC9, is unable to produce a decent shot.
I set off to create a pressurized portafilter to improve the coffee.
1) Cheap espresso machine
2) A rubber cap, such as will fit the bottom of the basket for the espresso machine (I used the cap off a pair of binoculars)
1) Swiss army knife
This can probably be adjusted to fit any low end espresso machine.
Step 1: Trim the Cap
It probably won't fit into the portafilter as is, trimming it to just barely climb the sides of the basket will allow it to be fitted into the portafilter.
Use part of the rubber you remove to create an o-ring to fit at the top of the basket to seal it to the portafilter.
Finally, poke a small hole in the middle of the rubber cap.
Step 2: Test the Machine
Check if the machine produces enough pressure to push the water through the hole. If necessary widen it slightly. Repeat until water gets out through the hole.
Note: the o-ring will make the portafilter harder to insert. Be careful not to damage anything, and if you see the portafilter isn't going in at all make the o-ring thinner.
Step 3: Enjoy Better Espresso
It won't be the quality you get at a cafe, but it will be better than you got on the machine previously.