Create Advantage: Make opposition more potent
In my own experience (somewhere about 20 games - mostly one-shots, though - GMed) the more potent the opposition, the more players are encouraged to make use of create advantage action. So, for instance, if a bad guy have his defence skill, which he uses to counter any attack action the players use, high enough to require them to use more than 2 fate points on average roll (Like, +6 defence vs +2 average player attack) just to make it a tie, then they'll probably start thinking about making those advantages. Same goes for high enough attack skills, that may deal significant harm to any PC.
I've had once a very interesting conflict, where PCs were facing a very strong enemy - mostly cybogized human with heavy X-COM-stylish plasma gun. He was also backed up by four henchmen. The PCs tried first to just shoot everything they saw only to realise they don't do enough damage by attacking alone (and suffer a few consequences while realising that) - henchmen are using very good cover (creating advantages for themselves) and main enemy is rather resistant to all their attacks - so they started to position themselves and making use of environment to prepare for decent strike. They end up making his henchmen to betray his master and then collapsed most of the ceiling on his head to win, one of the PCs was forced to take Extreme consequence and all of the players were mad happy about this encounter.
Tl;Dr version: For your players to start even considering using the create advantage action you must make sure they can't just shoot ot provoke everything they see without any consequences.
Side note: Be sure to prepare them somewhat before they engage such strong enemy. It's rather not fun to be used as a floor mop, when you haven't got any chance to prepare for it. My players in mentioned encounters were rather aware that the enemy will be quite strong, and in addition they let one of his henchmen escape in the previous encounter, so the baddie was also prepared for PCs.
Overcome: Give them a choice
As far as I remember, the only rule about overcomes in conflict in Fate Core rulebook is the time when you try to move more zones than 2, or something blocking your way and you must overcome it. So other than that, if you don't explicitly provide them with obstacles to move through zone, they don`t even need to overcome anything. So, for you to make them use this action, you have to either make any battlefield they encounter a war-torn bomb-and-shrapnel filled craterfield, or you could give them a choice.
By choice I mean, that they'll have some sort of reason to not just punch their enemy in the face (or make some arrangements for it - creating an advantage). It's usually not something that they can use to win over their opposition - that would be create an advantage action - but it must be something they might consider more important than winning. It might be a lock on the gate, that preventing them from escaping an endless zombie horde, or it might be some valuable shiny they might want to snatch before the pirate ship is utterly destroyed by their war mage, or whatever. It is, of course, might help them in the end to punch opposition in the face, like great artifact sword of awesomeness they spent 3 turns digging from debris, while other part of the party were fighting with incoming waves of goblins. But in general it must provide some sort of alternative goal they might have in addition to just defeat the opposition.
Tl;Dr version: Make something they will want to do other than punch their enemy in the face, and make getting that done difficult - hence, overcome action!
Side note: Of course, making some terrain hard to pass by, requiring overcome action is also a good thing if it provides some sort of tactical choice. My point, is that it is not the only one option to make overcome useful.
