In chess, some say that you learn more by reading books (i.e., learning and studying new tactics) than playing more games. I'm not sure whether this is true, however, I feel that to an extent, this applies well to D&D battle tactics.
Of course, to the best of my knowledge, there are aren't any books on D&D battle tactics and even if they were, you wouldn't expect your players to read books to play a game. So how to educate your players on tactics? This what I've done successfully:
Discuss their tactics after a session
In the first few sessions of the campaign I'm currently running my players were really bad at battle tactics. So what I did that, after each session, I broke down some their tactical choices and offered them alternatives.
Example: "You fought a Goblin Boss and two goblins. All four of you focused on the Goblin Boss and completely ignored the two goblins. On the surface, this sounds ok but the problem is that these puny monsters can accumulate a lot of damage over 5-6 turns. That's why half of you almost died. Generally speaking, a tactic that works most times is for the tank of the group to hold on to the boss while the rest clear the field of the mobs. Then, you all focus on the boss."
At first, I was unsure whether they will take my advice to heart, however, they did! And not only they used my advice, but they learned to synergize so well that in the last few sessions they surprised me by defeating some enemies who were clearly above their level.
Now, you don't explicitly mention that your players are bad at tactics, only that they use the same tactics all the time, but I don't see how this advice cannot be used to give them some ideas on being more creative.
Example: "Well done on defeating Monster X. You attacked it full-on, however, I was expecting you to take advantage of the barrels of oil that were a few meters away. If you had done so, you would have dealt Y damage points before even that battle had started!"
"Interesting choice attacking Z. You know, if you had simply asked him X he would probably have helped you and the village would be in favor of you. Now, the villagers hate you."
If you apply this method I believe that after a few sessions your player will get the idea. Being more creative is worth it.
Beyond my suggestion of simply suggesting things to them after a session, I use another method. This has worked with half of my players and deals specifically with battle tactics, not alternative ways of dealing with a problem (e.g., diplomacy). It's a homemade rule so maybe it's not for you. The rule is called:
This is Hollywood
Description: In battle, when attacking an opponent, instead of simply announcing your attack (e.g., I attack using my knife) describe your (elaborate) method in detail.
Example 1: "I jump over the balcony to the chandelier, backflip over the guard and aim at his neck with my knife! (requires an acrobatics roll)."
Example 2: "I focus on my opponent's defensive technique and attempt to find an opening. She may be heavily armed but this time my war hammer will make contact! (requires an Insight roll)"
Possible Effects: +2 to attack, epic moments, epic fails
This has done wonders with two of my players who enjoy being really flashy with their PCs. But the important thing is that it has made them more creative. In battle, they ask me to describe their surroundings and attempt to make use of them.