How to make a successful attack in Clash of Clans?

Choose a village with relatively weak defenses. You should do a few tests to know the strength of your current army to make sure that you are only attacking the ones weaker than you.

Some players expose their town-hall by putting it on the edge of the village or by even putting it out of the walls. This attack is very likely to succeed as taking down the town-hall alone is enough to make you win.

Don't attack a village if its town-hall level is higher than you. This could lead to finding more strong defenses than you are used to.

Put one unit near the enemies' castles to find out whether they have castle troops or not. If the troops showed up lure them away from the defenses by attacking a remote building. Take them out first before you complete the attack. (See How to Defeat Clan Castle Troops in Clash of Clans)

Sometimes your army might run out before destroying 50% of the enemy base, while a few remaining unprotected buildings could help you reach that 50%. By leaving a few units to the end you can take out those buildings easily.

When you upgrade your town hall before upgrading your troops you are very likely to have a weak army compared to the enemies you will attack. 

If it's possible to lure out heroes and kill them before beginning then do it. That will make your task much easier.

If the castle is placed in the center of a village in such a case that you can't reach it until the middle of the attack then don't attack this village; an unexpected unit might show up during the battle and kill your army. (See How to tell if Clan Castle is Full in Clash of Clans?)

Try not to advance in leagues fast in order to not find opponents you can't beat. If your score is going up then you should simply allow yourself to lose in order to lower it.

When attacking a base make sure that you have troops that can take it out in addition to some more troops. If you attack a base you can barely handle surprise traps might show up and ruin your attack. 

When you use your spells at the right time your chances of winning usually become higher. You should, for example, use the healing spell when your troops are low on health.

An unstable connection can result in connection termination during the attack. COC team recommends making an attack over a stable connection (preferably Wifi).