Use \def

\def is an incredibly powerful tool when it comes to golfing. \def allows you to assign something to a variable.

This can be used simply with the to save a number you may use a bunch of times for instance

\def\x{1456}

Will define 1456 as \x for future use much like saving a variable might in a programming language.

However \def is much more powerful than that, because \def doesn't define a variable it defines a snippet of code to be substituted into the program whenever it is called.

For example say you want to draw some rectangles using \draw you might write the following code:

\draw(0,0)rectangle(3,4)rectangle(8,0);\draw(2,2)rectangle(3,3);

Using \def this could be written as:

\def\x{)rectangle(}\draw(0,0\x3,4\x8,0);\draw(2,2\x3,3);

Use \tikz instead of the tikzpicture environment

Instead of creating a tikzpicture environment (36 bytes) you can use the \tikz command (7 bytes)

Global options can be set in square brackets in using the tikz command as such \tikz[options...]{...}. If the tikz code is one line long the curly braces can be omitted saving an additional two bytes.

Example:

Both of the following programs output the image at the bottom

\documentclass{standalone}\input tikz\begin{document}\tikz{\draw[thick,rounded corners=8pt](0,0)--(0,2)--(1,3.25)--(2,2)--(2,0)--(0,2)--(2,2)--(0,0)--(2,0);\draw(-1.5,0)--(0,1.5);}\end{document}

\documentclass{standalone}\input tikz\begin{document}\begin{tikzpicture}\draw[thick,rounded corners=8pt](0,0)--(0,2)--(1,3.25)--(2,2)--(2,0)--(0,2)--(2,2)--(0,0)--(2,0);\draw(-1.5,0)--(0,1.5);\end{tikzpicture}\end{document}

Credit to WheatWizard for figuring the multiline use of \tikz

Use \documentclass[tikz]{standalone}

By chance I found the following in the manual of the standalone package:

For pictures drawn with TikZ a dedicated tikz option is provided which loads the tikz package and also configures the tikzpicture environment to create a single cropped page.

Thus, instead of

\documentclass{standalone}\input tikz\begin{document} ...

one can write

\documentclass[tikz]{standalone}\begin{document} ...

to save 5 bytes.

You can sometimes refrain from using co-ordinate (0,0) completely.
That's an easy one.
Here's a more intricate one:
(,) is the same as (1,1).
Found out that one rather by accident.
Here's a minimal example:

\documentclass[tikz,border=9]{standalone}\def~{document}\begin~\tikz{\draw grid(,);}\end~

Output: