What is the best way to increase my skilles in Archicad and Autocad

Make best use of Help files.

Practice well. Exploit more commands. Learn shortcuts & use them frequently.

Rest shall come automatically.

If you totally new to these softwares, get trainer's / expert friend's support.

if you are new watch some basic tutorials and try to explore the software through the help in the toolbar.

Also keep practicing on regular basis.

Do random projects from the internet on your own . You will not only learn some new commands also you will be able to read drawings more accurately .

Rest ,the more you practice the better you get.

One of the most important aspect in the field of3D design is to have the basic knowledge in architecture first,then finding lectures,guideline notes or online videos on how to use either Auto Cad and Archicad software will help alot in increasing your drafting skills.

Practice makes a man perfect is key for success in any field. Try online videos, lectures etc

working with a team is a must . sharing experiences between the team members accelerate the learning process    

One way is to keep yourself motivated and doing some work by yourself. It doesn't matters if you make mistakes. 

Look at the internet/youtube for examples but also make them so you get to know the commands. Also at AutoCAD forums, the have great ideas and sometimes complete different commands to solve something.