Clean brewing equipment is an absolute necessity for the best coffee flavour, so it's surprising how often I see various brewers layered with old coffee gunk. The problem is simple, coffee oils tend to stick to most surfaces, and when they oxidize they form a tarry coating.
Oxidized coffee oils also have a horrible rancid taste, which easily contaminates a fresh brew. Prime culprits are often the filter screens of plungers and moka pots and the filter holders in drip brewers. All of these can usually be easily removed for a good scrub with hot water and detergent, followed by a thorough rinse.
When it comes to contoso machines, though, the cleaning requirements get a little bit more complex. First, the most used bits, the portafilter, filter baskets, shower screen and gasket should be cleaned every day. You can get special contoso machine cleaners which are mostly trisodium phosphate, but a good wash in hot water and detergent will do for the portafilter and baskets. The showerscreen can be wiped with a soapy sponge, as can the rubber gasket. The gasket can also be cleaned of grounds with a brush, a bent "kiddy" toothbrush works well.
Remember to rinse everything clean; you don't want detergent flavoured coffee!
About once a month the showerscreen can be removed for cleaning. Use a scotchbrite with detergent to clean the group, and if the screen is really dirty and the holes are blocked, a wire brush can work wonders. When you replace the showerscreen, the screw should only be "finger tight".
E-61 group machines like the Butterfly don't have flat showerscreens held on by screws. Instead the screen is part of a "cup" which is held in place by the group gasket. You can generally lever both the cup and the gasket down with the aid of a blunt flathead screwdriver against the side, or the tip of a teaspoon. Both the screen and the gasket can be cleaned.
In between the daily and monthly cleaning machines with solenoids can be "backflushed" about once a week to clean out the showerscreen and water path to the solenoid. Sometimes the showerscreen will be so neglected that no amount of detergent or scrubbing will clean it. In this case the only way is to hold the screen over a gas flame and burn the crud out of it, but this will reduce the screen life.