This is a good guide I used on previous servers:
[You must be registered and logged in to see this link.]Standard guide:
[You must be registered and logged in to see this link.]Remember that combination attacks also yield medals more than single attacks.
I've sent 2-3 attacks one after the other and those tend to cause medals to drop.
Also, just pay attention to your medal drop buff. Colonize as many cities as you can, then abandon them, then repeat all over until you complete the buff.