Time trials are an important event, and like all aspects of cycling, you only get better through practice.
I have found that time trialing at the highest level is a component of three major things. I mentioned the first one already--practice--and the other two are comfort and focus.
If you practice enough in your time trial position, you will get stronger, learn to recruit all of your leg muscles and be able to push yourself farther and farther. Practice time trials once a week if you really want to improve--this feeling of pushing yourself at your edge will become familiar to you.
It's very important that you do not start out too fast. Most athletes will start out too fast and then end up riding slower and slower toward the end. You should start out the first 10 minutes at a pace that's just a little bit slower than you know you can do, and then gradually pour on the power throughout the time trial. That's the correct pace.
You see, since you are so excited about the time trial, your perceived exertion is lower than normal for your workload in the beginning. Your adrenalin is pumping, you are ready to go, and you just launch out of the starting gate, only to have the reality of your too-high workload hit you about 10 minutes down the road.
In that beginning stage of the time trial, keep telling yourself, "Hold back, hold back." This is where a power meter could come in handy to really help you hold a certain wattage.
Now, once you have been practicing and you have become familiar with pushing yourself at the edge of your workload, then you become comfortable. Comfort has a lot to do with position on your bike, and as you practice more, you refine your position further, so that you are more comfortable.
Comfort in your aerodynamic time-trial position is VERY important. Too many people think they can slap on the aero bars, slam the saddle forward and lower the stem the day before their time trial and be ready to produce a peak performance--good luck!
Only through practicing your time trialing in your time trial position will you be ready for a peak performance. You must feel that your time trial position is like an old pair of jeans: When you get on your bike, you just fit into that familiar space that is routine for you. WHY? Because you will be comfortable there, which leads to the third and most important component of time trialing:
If you can stay focused on pushing your body to the edge and recruiting all your muscles in your legs--not just the hamstrings and quads--then you can produce a peak performance. However, this is incredibly tough and you have to be physically and mentally ready to push yourself at your highest level for a fairly long period of time.
If you can produce the same feelings in all of the leg muscles as you can in your quads, then you will be focused on recruiting those muscles and turning that into speed on the bike.
Focus means staying in the present, in the moment, and nothing else matters. This is not an easy thing to do in a time trial; it's easier in a tough criterium race, when you are on the front and attacking or in a breakaway with others.
For focus, you must concentrate on your breathing rhythm, the sensations in the legs, and nothing else. If you find yourself looking at the trees, the cars going by, the pretty houses on the side of the road, you are not focused. This is just a trick of your mind to get you out of your limit, away from that edge: "Hey, this stuff is tough, it hurts, I don't know if I can do this... " and then you ease off of the edge and start to lose your focus--and that's when you let go of your possibility of a peak performance.
Focus is the most important part of the experience, allowing you to push yourself to a winning time. Rest assured, it doesn't happen every time, but if you work on the first two components--practice and comfort--that razor-sharp focus will be easier to achieve for the entire effort.