Here are are some tips for making a EV3 robot have a good design and programs. This can be used to help achieve a good score in a FLL competition, or to just have fun with some friends.

Step 1: Mechanical Design Tips

Here are some mechanical design tips for you robot to make it work well and smoothly:

It is important to make sure that there is a lot of spaces to allow for lots of attachments. This allows for more than one attachment to be put on at once, and that allows for more missions or things to be done at once.
Motors are attached cleanly to the robot so that if the attachment is pulled off it will not pull off the motors.
The robot is sturdy and it does not fall apart if it falls or if it is bumped. This allows the robot to attempt missions that can cause the robot to fall.
If the wheels on the robot are small, there will be less of a chance for the robot to slip which would cause the mission that the robot was doing to fail.
Color sensors are also very useful as they allow you to follow lines to get to places easily.
Gyro sensors can allow you to turn exact degrees and drive forward, so if the robot turns it turns back so it always drives to the same place.
Making a square robot allows you to bump into walls and to square, and the walls of the robot keep wires in.
One final important tip is to put the center of gravity, or the brick in the center of the robot.

Step 2: Programming Tips for the Lego Mindstorms Programming Software

Here are some programming tips for your robot:

Make Myblocks, because they can be used to make the robot move a certain number of inches or turn a certain amount of degrees, so you can measure everything on the board, program it in, and run the program.
Write comments in your program so that if you come back later to fix it, you can understand what everything does.
Use lots of line followers. Though they are slow, they get you to places in about the same place every time.
Gyro turn programs help make everything very precise and accurate, with you only needing to know the exact amount of degrees.
Using Sudo code, or writing out exactly what you are going to do in your program is very helpful.
As always, the simpler, the better.
When you are testing, make sure that what you are doing works 3 times in a row before calling it done.