First, let's talk about some techniques that without you writing any code.
1) Less is More
2) Where to place your "script" tag
3) Dynamic download or Loading on Demand
4) Avoid using global variable
Make sure you use "var" when you declare a variable, or else it will become a global variable. And it is slow. The reason is because the interpreter needs to search for where your variable is declared in the code if it is global. Plus, a global variable have other potential problems besides speed, for example, namespace collision.
5) Cache your variable as needed
For example, if you need to access an array length in a loop, instead of calling "arrayX.length" everytime, you can cache that value in a variable, so there is no need to search for that value in every loop. In the same token, if you have a value or object needs to be accessed for multiple time, and if that value / object won't be changed, it is a good candidate to be cached.
6) Understand Delegation
For example, you have an unordered list of 100 links. Instead of binding each links a "click" event, you can just bind the click event to its parent, which is only 1 item. This way, it saves you coding time, and save the parse to search the DOM for 100 times. And that is what called Delegation.
7) Code Reusability
Understand how to reuse your code will definitely makes your code cleaner and you are writing less code. For starter, let's give you some keywords in this post. I will write another post on those later.
a) Inheritance - either Classical or Prototypal.
b) Borrowing other object method ( call, apply, bind )