Biggest problem I see with front kicks, and I used to do this, is chambering the leg like a Karate front kick. Remember, in Muay Thai and (effective) kick boxing, the idea is to use the power of your hips; not your quadricep. In Muay Thai sparring, I like to use the teep a lot, but the only time I land it is when I properly do it; that is, not chambering the leg and using the power of my hips. It's much faster and harder to see coming.
Primarily, I'd use it in MMA as a defensive move. And it can only be used once in a while. When you start throwing a lot of them, that's when you get in trouble. But really, his momentum coming towards you, plus the momentum of your front leg teep, aimed towards the solar plexus, will really catch him off guard. Good Thai boxers and kickboxers will catch you with these out of nowhere.
I'll just list some good ideas for the teep. I'm not a MT expert, but I love to spar, and I still like to study Muay Thai a lot. Try these out in sparring and let me know what works.
-Fake a roundhouse kick, as you bring the leg around, switch off to a rear leg teep kick. Got this from Bas Rutten. He used it a lot in Pancrase with a high percentage of success.
-Aim teeps to the solar plexus/diaphragm (the pocket under the sternum). I got caught with one of these lightly; it is not fun. Saw Sem Schilt KO a guy with a front snap kick to the solar plexus; a teep will do just as much damage IMO
-Aim teeps to the legs. Especially around the knees. When both of his feet are on the ground, use your heels instead of the ball of your foot, and aim them above the knees. Not a KO move, but can be as upsetting as a good roundhouse kick to the sciatic nerve.
-Aim teeps to the chest. Bas would do this sometimes. If you hit him high on the chest hard enough, you'll throw him off balance. If you're fighting in a ring, do it near the ropes, and he'll fly into them. Be careful though; good Thai boxers will bounce off the ropes and use their momentum to retaliate with an elbow or a punch. It might be good to do this one in the corner instead.
-Aim teeps to the face. Very difficult, but I've seen Ramon Dekkers do this. This is a dangerous one; dangerous to the kicker moreso than the receiving end. It's so easy to miss, and end up with your leg on his shoulder. Not a good position. Wait until he starts approaching you, and aim a good teep to the face. Believe me, if you land it right, this will really upset and distract him.
-Use the teep when he attacks. This is my favorite way to use it. When he's totally committed to an attack, his mind isn't on catching your leg. And think about it. In Muay Thai, the teep is your longest weapon. No weapon he has; punches, round kicks, elbows, knees, or clinching, can reach you if he's at the end of your teep. And other than the jab, every attack in boxing and kickboxing temporarily throws you out of balance (that's why the jab is used more than any other punch; it doesn't compromise your weight distribution). So when you throw the teep in the middle of the attack, he's already off balance, and now he's probably going to hit the floor. When he throws a punch or long knee, teep kick him to the solar plexus. A hard teep there will really upset him, and if you're quick, you can follow up with a combination while he's distracted. When he throws a round kick, this is, IMO, the absolute best time to use the teep. Kick him to the solar plexus or chest to send him flying backwards. You can kick the leg that he's kicking with, and it will check his kick (meaning it will stop it). You can also kick the pivoting leg, which is not a bad idea.
-Use the teep as a "foot jab." This is kind of a dangerous one in MMA; you don't see it a lot. If you get predictable, your leg will get caught. But it can serve as a good distraction. A good combination is a front leg teep-long right knee-pivot step-right straight.
You just need to set it up properly. And my suggestion: if you're using it in an actual fight, don't dick around with it. Why would you ever throw a half-assed strike? With proper conditioning, you should be able to throw strikes at least 90% power, with some 100% flurries. Half-assed strikes are the ones that get caught or countered. So when you throw the teep, be serious about it. Really drive your hips and try to pierce through the target. I learned through some San Shou that the proper way to catch a thrusting kick is to step out the side first. I rarely see people do this when they catch a teep. If you throw it hard, and he's sloppy with the catch, you can still connect.
And hey; since when does having your leg caught mean you're going to get taken down? If your leg gets caught when you teep, or roundhouse, here are some good options.
-Immediately wind up and throw bombs at his face. The proper way to catch a kick is to immediately, or simultaneously punch and/or throw him off balance. In a fight, your opponent won't always remember this. If he even offers you an inch of his face, or a fraction of a second before he follows up with a takedown or a punch, unload on him. Don't let the guy even put his head up.
-After doing the above, or if he hasn't taken you down yet, grab onto his head, and flying knee to the face or chest. He's holding on to your leg, so even if you aren't the most athletic person in the world, trust me, you'll be able to get high enough off of the ground.
-Downward elbow strike to the collarbone. Yeah. I know. Probably illegal in most competitions. Technically it would be legal in Pride, wouldn't it? UFC says no dowward elbows, but Pride only says no elbows to the head, if I'm not mistaken. And if you compete in Thai boxing, in some places, elbows are legal. Collarbones break easy guys. If you break his collarbone, the arm on that side is pretty much useless. And with only one arm he's pretty screwed.
-Underhook the arm that he has your leg with (if he has your right leg with his left arm, then underhook his left arm with your right arm), and get really close. Push up with your underhook, and push down with your leg. Good way to break his grip.
-Jump roundhouse to the head. I have never seen this, and never done it, but it's just an idea that I have. It most likely will not be a KO shot. But it will be a big distraction, and if done hard enough, it should be enough to get him to let go of your leg. Just jump up, twist your body around, and try to hit him in the head with your leg. Get ready to hit the ground and get up as fast as possible.
The trick is, when he has your leg, that he only has power over your leg if he's grabbing the ankle or close to it. The closer he is to your knee or butt, the more power you have. So if he caught the kick properly, and he has your ankle or heel, just get really close to him, and slide your leg deeper. If you can get his hand to right above the pit of your knee, trust me, he will have no power. And if you use the underhook trick in tandem with that; he'd have to be Mariusz Pudzianowski to be able to keep holding on to your leg.
Hope this helped anyone who was having trouble with the teep. It's a good kick, but it just has to be thrown rarely, and when it is thrown, with good timing and power.