While it's true spending more time riding on the road instead of the indoor trainer will help, incorporating specific drills into your training will help, too.
To get started, try each of these four drills once a week.
Ride the Line1 of 5
Helps improve: balance, riding in straight line without swerving
How to practice: In your aerobars, ride on the white line close to the shoulder. See how far you can ride without touching the asphalt. This will give you an idea how straight you actually ride. The faster you go, the easier it will be. As you improve, try riding at a slower speed.
Water Bottle Pick-up2 of 5
Helps improve: balance in corners, shifting weight
How to practice: Place a few water bottles in an open area such as a soccer field. At a slow speed, ride by each bottle and lean over to pick it up as you pass. Practice this drill with your right hand first, then the left. As you gain confidence, move from the safety of the grass to the road.
The Criterium3 of 5
Helps improve: cornering confidence, maintaining speed during turns
How to practice: Find an empty parking lot and make a large square with four cones. Start out by riding around the square making only left-hand turns. You'll want to keep your speed as high as possible around each corner without hitting the brakes. As you approach each turn, swing wide and lean your bike into the corner, aiming to "cut the corner" by riding as close to the cone as possible. Remember, don't touch the brakes. Practice using your hips to steer the bike instead of turning the handlebars. As you exit the corner, pedal hard and accelerate to the next cone.
Once you think you've got it down, time yourself to see how fast you can make it around. When your left-hand turns are mastered, practice riding around the square in the opposite direction, making only right-hand turns.
The Bunny Hop4 of 5
Helps improve: safety on the road, avoiding objects such as potholes
How to practice: In parking lot, ride at a moderate pace toward one of the white lines that delineate parking spaces. As you near the line, pull up on your handlebars so that your front tire doesn't touch the paint. As you become comfortable getting your front wheel in the air, practice lifting your rear wheel at the same time by using your legs to pull up on the pedals (your feet should be in the three o'clock and nine o'clock position of the pedal stroke).
Do this until you can lift both the front and rear wheel off the ground at the same time. This skill can keep you from riding into large potholes or help you to ride up a curb in the case of an emergency.