Home espresso machines vary greatly, but here are some general guidelines to make a good espresso drink. 

1. If your machine is not hooked to a water supply, be sure to use fresh, filtered water.
2. Grind your coffee fresh, right before you brew.  Try to grind so that a double shot of espresso takes about 20-30 seconds to pour. 
3. Keep your machine clean.  Don't leave coffee in the portafilter, remove the basket and scrub with a scotchbrite pad and warm water.  A dirty machine will give you harsh, bitter coffee.