Remove trees and stones.
Remove trees, stones and other objects that show up near your village. In many cases you will find gems inside them.
Remove the gem box
The gem box, which sometimes appears near your village, has many gems. Many sure to remove it as soon as you find it.
Complete the Achievements
The achievements list includes many missions that can bring you gems when you complete them. Make sure you complete as many as you can.
Win Multiplayer battles
When you keep winning Multiplayer battles you keep getting league points , the ones beside the image of a cup. Those points can help you get a large amount of gems. For example when that number reaches 1250 you get 450 free gems.
Keep upgrading your village
Many of the achievements that bring gems are just simple upgrades to your buildings. Make sure you keep upgrading. (See How to make a successful attack in Clash of Clans?)
Don’t lose multiplayer battles
Pick your multiplayer battles battles wisely. Losing a battle will result in reducing the number of league points you got and so it will become harder to get large batches of gems.
Use gems to get builder huts
Don’t use the gems for anything but builder huts. By doing so you will be able to accelerate the process of getting more gems since you will lots of active builders. (See Why you should use decorations in Clash of Clans?)
How to get free gems in clash of clans?
How do you get free gems on clash of clans? Best Way to get free gems in clash of clans