I agree with Princess Aurora, go early, get fast passes while you ride standby until the lines get longer, then use your fastpasses. Also, try to get another fastpass as soon as the window opens on your last fastpass.
Also, consider going early or at park opening to the park that had extra magic evening hours the night before. Everyone may just still be in bed.
When we checked in to our resort, they gave us a list of the parks that had EMH in the morning and evening for the time we were there. It also listed the attractions that were open for the EMH times. There were fewer attractions open in the am. Not many open period at Epcot or MGM. Because so few attractions were open, the lines seemed long.
Also, you have to show your resort card to get in to am EMH. For the evening EMH, you have to get a ride bracelet. You can get the bracelet no earlier than 2 hours before regular park closing. There are several locations to get the bracelet at each park. The lines at the location at the front of the parks are usually long. Pick another location if possible. The lines may be shorter after the EMH evening starts. The CM's are only supposed to let guest with bracelets ride the attractions. However, I don't believe guests who are not staying in a Disney resort are required to leave the parks (they can still shop, eat, wander around, etc.), so the parks may seem crowded.
We were still able to use our fast passes from earlier at some rides during evening EMH, but not all. Fast passes were still available for a short while as well at some rides. It varied greatly.